BUG: Series mode crashes when there is more than one mode
- [ x] I have checked that this issue has not already been reported.

- [ x] I have confirmed this bug exists on the latest version of pandas.

- [ ] (optional) I have confirmed this bug exists on the master branch of pandas.

---

**Note**: Please read [this guide](https://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports) detailing how to provide the necessary information for us to reproduce your bug.

#### Code Sample, a copy-pastable example

```python
import pandas as pd
pd.DataFrame({"data" : [1,1], "value":[0,1]}).groupby("data").agg({"value":pd.Series.mode})
```

#### Problem description

This line of code crashes with `ValueError: Must produce aggregated value`

<details>

```python
Traceback (most recent call last):
  File "/usr/local/Caskroom/miniconda/base/envs/rapids202012/lib/python3.7/site-packages/pandas/core/groupby/generic.py", line 261, in aggregate
    func, *args, engine=engine, engine_kwargs=engine_kwargs, **kwargs
  File "/usr/local/Caskroom/miniconda/base/envs/rapids202012/lib/python3.7/site-packages/pandas/core/groupby/groupby.py", line 1085, in _python_agg_general
    result, counts = self.grouper.agg_series(obj, f)
  File "/usr/local/Caskroom/miniconda/base/envs/rapids202012/lib/python3.7/site-packages/pandas/core/groupby/ops.py", line 658, in agg_series
    return self._aggregate_series_pure_python(obj, func)
  File "/usr/local/Caskroom/miniconda/base/envs/rapids202012/lib/python3.7/site-packages/pandas/core/groupby/ops.py", line 717, in _aggregate_series_pure_python
    raise ValueError("Function does not reduce")
ValueError: Function does not reduce

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/Caskroom/miniconda/base/envs/rapids202012/lib/python3.7/site-packages/pandas/core/groupby/generic.py", line 951, in aggregate
    result, how = self._aggregate(func, *args, **kwargs)
  File "/usr/local/Caskroom/miniconda/base/envs/rapids202012/lib/python3.7/site-packages/pandas/core/base.py", line 416, in _aggregate
    result = _agg(arg, _agg_1dim)
  File "/usr/local/Caskroom/miniconda/base/envs/rapids202012/lib/python3.7/site-packages/pandas/core/base.py", line 383, in _agg
    result[fname] = func(fname, agg_how)
  File "/usr/local/Caskroom/miniconda/base/envs/rapids202012/lib/python3.7/site-packages/pandas/core/base.py", line 367, in _agg_1dim
    return colg.aggregate(how)
  File "/usr/local/Caskroom/miniconda/base/envs/rapids202012/lib/python3.7/site-packages/pandas/core/groupby/generic.py", line 267, in aggregate
    result = self._aggregate_named(func, *args, **kwargs)
  File "/usr/local/Caskroom/miniconda/base/envs/rapids202012/lib/python3.7/site-packages/pandas/core/groupby/generic.py", line 482, in _aggregate_named
    raise ValueError("Must produce aggregated value")
ValueError: Must produce aggregated value
```
#### Expected Output
`[0,1]`

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : b5958ee1999e9aead1938c0bba2b674378807b3d
python           : 3.7.9.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 20.1.0
Version          : Darwin Kernel Version 20.1.0: Sat Oct 31 00:07:11 PDT 2020; root:xnu-7195.50.7~2/RELEASE_X86_64
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.1.5
numpy            : 1.19.2
pytz             : 2020.4
dateutil         : 2.8.1
pip              : 20.3.3
setuptools       : 51.0.0.post20201207
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.2
IPython          : None
pandas_datareader: None
bs4              : None
bottleneck       : None
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pyxlsb           : None
s3fs             : None
scipy            : 1.5.2
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
numba            : None

</details>
