Retrieve the PyTorch model from a PyTorch lightning model
I have trained a PyTorch lightning model that looks like this:
In [16]: MLP
Out[16]:
DecoderMLP(
  (loss): RMSE()
  (logging_metrics): ModuleList(
    (0): SMAPE()
    (1): MAE()
    (2): RMSE()
    (3): MAPE()
    (4): MASE()
  )
  (input_embeddings): MultiEmbedding(
    (embeddings): ModuleDict(
      (LCLid): Embedding(5, 4)
      (sun): Embedding(5, 4)
      (day_of_week): Embedding(7, 5)
      (month): Embedding(12, 6)
      (year): Embedding(3, 3)
      (holidays): Embedding(2, 1)
      (BusinessDay): Embedding(2, 1)
      (day): Embedding(31, 11)
      (hour): Embedding(24, 9)
    )
  )
  (mlp): FullyConnectedModule(
    (sequential): Sequential(
      (0): Linear(in_features=60, out_features=435, bias=True)
      (1): ReLU()
      (2): Dropout(p=0.13371112461182535, inplace=False)
      (3): LayerNorm((435,), eps=1e-05, elementwise_affine=True)
      (4): Linear(in_features=435, out_features=435, bias=True)
      (5): ReLU()
      (6): Dropout(p=0.13371112461182535, inplace=False)
      (7): LayerNorm((435,), eps=1e-05, elementwise_affine=True)
      (8): Linear(in_features=435, out_features=435, bias=True)
      (9): ReLU()
      (10): Dropout(p=0.13371112461182535, inplace=False)
      (11): LayerNorm((435,), eps=1e-05, elementwise_affine=True)
      (12): Linear(in_features=435, out_features=435, bias=True)
      (13): ReLU()
      (14): Dropout(p=0.13371112461182535, inplace=False)
      (15): LayerNorm((435,), eps=1e-05, elementwise_affine=True)
      (16): Linear(in_features=435, out_features=435, bias=True)
      (17): ReLU()
      (18): Dropout(p=0.13371112461182535, inplace=False)
      (19): LayerNorm((435,), eps=1e-05, elementwise_affine=True)
      (20): Linear(in_features=435, out_features=435, bias=True)
      (21): ReLU()
      (22): Dropout(p=0.13371112461182535, inplace=False)
      (23): LayerNorm((435,), eps=1e-05, elementwise_affine=True)
      (24): Linear(in_features=435, out_features=435, bias=True)
      (25): ReLU()
      (26): Dropout(p=0.13371112461182535, inplace=False)
      (27): LayerNorm((435,), eps=1e-05, elementwise_affine=True)
      (28): Linear(in_features=435, out_features=435, bias=True)
      (29): ReLU()
      (30): Dropout(p=0.13371112461182535, inplace=False)
      (31): LayerNorm((435,), eps=1e-05, elementwise_affine=True)
      (32): Linear(in_features=435, out_features=435, bias=True)
      (33): ReLU()
      (34): Dropout(p=0.13371112461182535, inplace=False)
      (35): LayerNorm((435,), eps=1e-05, elementwise_affine=True)
      (36): Linear(in_features=435, out_features=1, bias=True)
    )
  )
)

I need the corresponding PyTorch model to use in one of my other applications.
Is there a simple way to do that?
I thought of saving the checkpoint but then I don't know how to do it.
Can you please help?
Thanks

You can manually save the weights of the torch.nn.Modules in the LightningModule. Something like:
trainer.fit(model, trainloader, valloader)

torch.save(
    model.input_embeddings.state_dict(),
    &quot;input_embeddings.pt&quot;
)
torch.save(model.mlp.state_dict(), &quot;mlp.pt&quot;)

Then to load without needing Lightning:
# create the &quot;blank&quot; networks like they
# were created in the Lightning Module
input_embeddings = MultiEmbedding(...)
mlp = FullyConnectedModule(...)

# Load the models for inference
input_embeddings.load_state_dict(
    torch.load(&quot;input_embeddings.pt&quot;)
)
input_embeddings.eval()

mlp.load_state_dict(
    torch.load(&quot;mlp.pt&quot;)
)
mlp.eval()

For more information about saving and loading PyTorch Modules see Saving and Loading Models: Saving &amp; Loading Model for Inference in the PyTorch documentation.
Since Lightning automatically saves checkpoints to disk (check the lightning_logs folder if using the default Tensorboard logger), you can also load a pretrained LightningModule and then save the state dicts without needing to repeat all the training. Instead of calling trainer.fit in the previous code, try
model = DecoderMLP.load_from_checkpoint(&quot;path/to/checkpoint.ckpt&quot;)

