Can't iterate through PyTorch DataLoader
I am trying to learn PyTorch and create my first neural network. I am using a custom dataset, here is a sample of the data:
ID_REF  cg00001854  cg00270460  cg00293191  cg00585219  cg00702638  cg01434611  cg02370734  cg02644867  cg02879967  cg03036557  cg03123104  cg03670302  cg04146801  cg04570540  cg04880546  cg07044749  cg07135408  cg07303143  cg07475178  cg07553761  cg07917901  cg08016257  cg08548498  cg08715791  cg09334636  cg11153071  cg11441796  cg11642652  cg12256803  cg12352902  cg12541127  cg13313833  cg13500819  cg13975075  cg14061946  cg14086922  cg14224196  cg14530143  cg15456742  cg16230982  cg16734549  cg17166941  cg17290213  cg17292667  cg18266594  cg18335535  cg18584803  cg19273773  cg19378199  cg19523692  cg20115827  cg20558024  cg20608895  cg20899581  cg21186299  cg22115892  cg22454769  cg22549547  cg23098693  cg23193759  cg23500537  cg23606718  cg24079702  cg24888989  cg25090514  cg25344401  cg25635000  cg25726357  cg25743481  cg26019498  cg26647566  cg26792755  cg26928195  cg26940620  Age
0   0.252486    0.284724    0.243242    0.200685    0.904132    0.102795    0.473919    0.264084    0.367480    0.671434    0.075955    0.329343    0.217375    0.210861    1.000000    0.356048    0.577945    0.557148    0.249014    0.847134    0.254539    0.319858    0.220589    0.796789    0.361994    0.296101    0.105965    0.239796    0.169738    0.357586    0.365674    0.132575    0.250932    0.283227    1.000000    0.262259    0.208146    0.290623    0.113049    0.255710    0.555382    0.281046    0.168826    0.492007    0.442871    0.509569    0.219183    0.641244    0.339088    0.164062    0.227678    0.340220    0.541491    0.423010    0.621303    0.243750    0.869947    0.124120    0.317660    0.985243    0.645869    0.590888    0.841485    0.825372    0.904037    0.407343    0.223722    0.352113    0.855653    0.289593    0.428849    0.719758    0.800240    0.473586    68
1   0.867671    0.606590    0.803673    0.845942    0.086222    0.996915    0.871998    0.791823    0.877639    0.095326    0.857108    0.959701    0.688322    0.650640    0.062329    0.920434    0.687537    0.193038    0.891809    0.273775    0.583457    0.793486    0.798427    0.102910    0.773496    0.658568    0.759050    0.754877    0.787817    0.585895    0.792240    0.734543    0.854528    0.735642    0.389495    0.736709    0.600386    0.775989    0.819579    0.696350    0.110374    0.878199    0.659849    0.716714    0.771206    0.870711    0.919629    0.359592    0.677752    0.693433    0.683448    0.792423    0.933971    0.170669    0.249908    0.879879    0.111498    0.623053    0.626821    0.000000    0.157429    0.197567    0.160809    0.183031    0.202754    0.597896    0.826429    0.886736    0.086038    0.844088    0.761793    0.056548    0.270670    0.940083    21
2   0.789439    0.594060    0.857086    0.633195    0.000000    0.953293    0.832107    0.692119    0.641294    0.169303    0.935807    0.674698    0.789146    0.796555    0.208590    0.791318    0.777537    0.221895    0.804405    0.138006    0.738616    0.758083    0.749127    0.180998    0.769312    0.592938    0.578885    0.896125    0.553588    0.781393    0.898768    0.705339    0.861029    0.966552    0.274496    0.575738    0.490313    0.951172    0.833724    0.901890    0.115235    0.651489    0.619196    0.760758    0.902768    0.835082    0.610065    0.294962    0.907979    0.703284    0.775867    0.910324    0.858090    0.190595    0.041909    0.792941    0.146005    0.615639    0.761822    0.254161    0.101765    0.343289    0.356166    0.088915    0.114347    0.628616    0.697758    0.910687    0.133282    0.775737    0.809420    0.129848    0.126485    0.875580    20
3   0.615803    0.710968    0.874037    0.771136    0.199428    0.861378    0.861346    0.695713    0.638599    0.158479    0.903668    0.758718    0.581146    0.857357    0.307756    0.977337    0.805049    0.188333    0.788991    0.312119    0.706578    0.782006    0.793232    0.288111    0.691131    0.758102    0.829221    1.000000    0.742666    0.897607    0.797869    0.803221    0.912101    0.736800    0.315636    0.760577    0.609101    0.733923    0.578598    0.796944    0.096960    0.924135    0.612601    0.727117    0.905177    0.776481    0.727865    0.429820    0.666803    0.924595    0.567474    0.752196    0.742709    0.303662    0.168286    0.720899    0.099313    0.595328    0.734024    0.268583    0.293437    0.244840    0.311726    0.213415    0.418673    0.819981    0.816660    0.684730    0.146797    0.686270    0.777680    0.087826    0.335125    1.000000    23
4   0.847329    0.735766    0.858018    0.896453    0.186994    0.831964    0.762522    0.840186    0.830930    0.199264    0.788487    0.912629    0.702284    0.838771    0.065271    0.959230    0.912387    0.377203    0.794480    0.207909    0.766246    0.582117    0.902944    0.301144    0.765401    0.715115    0.646735    0.812084    0.697886    0.714310    0.890658    0.826644    0.944022    0.729517    0.530379    0.756268    0.764899    0.914573    0.825766    0.673394    0.017316    0.949335    0.614375    0.650553    0.898788    0.685396    0.823348    0.210175    0.831852    0.829067    0.858212    0.916433    0.778864    0.241186    0.144072    0.889536    0.058360    0.703567    0.852496    0.094223    0.341236    0.284903    0.231957    0.125196    0.333207    0.752592    0.899356    0.839006    0.174601    0.937948    0.716135    0.000000    0.114062    0.969760    22

I split the data into train/test/val data like this:
train_df, rest_df = train_test_split(df, test_size=0.4)
test_df, val_df = train_test_split(rest_df, test_size=0.5)

x_train_tensor = torch.tensor(train_df.drop('Age', axis=1).to_numpy(), requires_grad=True)
y_train_tensor = torch.tensor(train_df['Age'].to_numpy())

x_test_tensor = torch.tensor(test_df.drop('Age', axis=1).to_numpy(), requires_grad=True)
y_test_tensor = torch.tensor(test_df['Age'].to_numpy())

x_val_tensor = torch.tensor(val_df.drop('Age', axis=1).to_numpy(), requires_grad=True)
y_val_tensor = torch.tensor(val_df['Age'].to_numpy())

bs = len(train_df.index)//10
train_dl = DataLoader(train_df, bs, shuffle=True)
test_dl = DataLoader(test_df, len(test_df), shuffle=False)
val_dl = DataLoader(val_df, bs, shuffle=False)

And here is the Network so far (very basic, just to test if it works):
class Net(nn.Module):
    def __init__(self):
        super().__init__()
        input_size = len(df.columns)-1
        self.fc1 = nn.Linear(input_size, input_size//2)
        self.fc2 = nn.Linear(input_size//2, input_size//4)
        self.fc3 = nn.Linear(input_size//4, 1)

    def forward(self, x):
        x = F.relu(self.fc1(x))
        x = F.relu(self.fc2(x))
        x = F.relu(self.fc3(x))

        return x

net = Net()
print(net)

Here is where I get the error, on the last line:
loss = torch.nn.MSELoss()
optimizer = optim.Adam(net.parameters(), lr=0.001)

EPOCHS = 3
STEPS_PER_EPOCH = len(train_dl.dataset)//bs
iterator = iter(train_dl)
print(train_dl.dataset)
for epoch in range(EPOCHS):
    for s in range(STEPS_PER_EPOCH):
        print(iterator)
        iterator.next()

ID_REF  cg00001854  cg00270460  cg00293191  ...  cg26928195  cg26940620  Age
29        0.781979    0.744825    0.744579  ...    0.242138    0.854054   19
44        0.185400    0.299145    0.160084  ...    0.638449    0.413286   69
21        0.085470    0.217421    0.277675  ...    0.863455    0.512334   75
4         0.847329    0.735766    0.858018  ...    0.114062    0.969760   22
20        0.457293    0.462984    0.323835  ...    0.584259    0.481060   68
33        0.784562    0.845031    0.958335  ...    0.122210    0.854005   19
25        0.258434    0.354822    0.405620  ...    0.677245    0.540463   70
27        0.737131    0.768188    0.897724  ...    0.203228    0.831175   20
37        0.002051    0.202403    0.134198  ...    0.753844    0.302229   70
10        0.737427    0.537413    0.614343  ...    0.464244    0.723953   23
0         0.252486    0.284724    0.243242  ...    0.800240    0.473586   68
32        0.927260    1.000000    0.853864  ...    0.261990    0.892503   18
7         0.035825    0.271602    0.236109  ...    1.000000    0.471256   69
17        0.000000    0.202986    0.132144  ...    0.874550    0.342981   79
18        0.282112    0.479775    0.218852  ...    0.908217    0.426143   79
11        0.708797    0.536074    0.721171  ...    0.048768    0.699540   27
15        0.686921    0.639198    0.858981  ...    0.305142    0.978350   24
38        0.246031    0.186011    0.235928  ...    0.754013    0.342380   70
30        0.814767    0.771483    0.437789  ...    0.000000    0.658354   18
43        0.247471    0.399231    0.271619  ...    0.895016    0.468336   72
46        0.000428    0.263164    0.163303  ...    0.567005    0.252806   76
3         0.615803    0.710968    0.874037  ...    0.335125    1.000000   23
5         0.777925    0.821814    0.636676  ...    0.233359    0.753266   20
34        0.316262    0.307535    0.203090  ...    0.570755    0.351226   73
23        0.133038    0.000000    0.208442  ...    0.631202    0.459593   76
6         0.746102    0.585211    0.626580  ...    0.311914    0.753994   25
1         0.867671    0.606590    0.803673  ...    0.270670    0.940083   21
47        0.444606    0.502357    0.207560  ...    0.987106    0.446959   71

[28 rows x 75 columns]
&lt;torch.utils.data.dataloader._SingleProcessDataLoaderIter object at 0x7f166241c048&gt;
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
/usr/local/lib/python3.6/dist-packages/pandas/core/indexes/base.py in get_loc(self, key, method, tolerance)
   2645             try:
-&gt; 2646                 return self._engine.get_loc(key)
   2647             except KeyError:

pandas/_libs/index.pyx in pandas._libs.index.IndexEngine.get_loc()

pandas/_libs/index.pyx in pandas._libs.index.IndexEngine.get_loc()

pandas/_libs/hashtable_class_helper.pxi in pandas._libs.hashtable.PyObjectHashTable.get_item()

pandas/_libs/hashtable_class_helper.pxi in pandas._libs.hashtable.PyObjectHashTable.get_item()

KeyError: 13

During handling of the above exception, another exception occurred:

KeyError                                  Traceback (most recent call last)
6 frames
/usr/local/lib/python3.6/dist-packages/pandas/core/indexes/base.py in get_loc(self, key, method, tolerance)
   2646                 return self._engine.get_loc(key)
   2647             except KeyError:
-&gt; 2648                 return self._engine.get_loc(self._maybe_cast_indexer(key))
   2649         indexer = self.get_indexer([key], method=method, tolerance=tolerance)
   2650         if indexer.ndim &gt; 1 or indexer.size &gt; 1:

pandas/_libs/index.pyx in pandas._libs.index.IndexEngine.get_loc()

pandas/_libs/index.pyx in pandas._libs.index.IndexEngine.get_loc()

pandas/_libs/hashtable_class_helper.pxi in pandas._libs.hashtable.PyObjectHashTable.get_item()

pandas/_libs/hashtable_class_helper.pxi in pandas._libs.hashtable.PyObjectHashTable.get_item()

KeyError: 13

I really have no idea what the error means or where to look.
I'd greatly appreciate some guidance, thank you!

Use Numpy array instead of dataframe. You can use to_numpy() to convert dataframe to numpy array.
train_dl = DataLoader(train_df.to_numpy(), bs, shuffle=True)
test_dl = DataLoader(test_df.to_numpy(), len(test_df), shuffle=False)
val_dl = DataLoader(val_df.to_numpy(), bs, shuffle=False)

