BUG: Should be able to group by a categorical Series of unequal length
### 

- [X] I have checked that this issue has not already been reported.

- [X] I have confirmed this bug exists on the [latest version](https://pandas.pydata.org/docs/whatsnew/index.html) of pandas.

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


### Reproducible Example

```python
import pandas as pd
series = pd.Series(range(7))
grouper = pd.Series(list("ABBA"), dtype="category")
series.groupby(grouper)

# Raises ValueError: Length of grouper (4) and axis (7) must be same length
```


### Issue Description

When grouping by a `Series`, the values of the series after alignment determine the groups. In the example, however, pandas does not attempt to align the series and instead unnecessarily wants it to have the same length as the axis of grouping.

### Expected Behavior

I expect `obj.groupby(grouper)`, where `grouper` is a `Series` with a categorical data type, to allow the grouper to have unequal length and to align it, as in the following where the grouper has a non-categorical data type:

```python
series.groupby(grouper.astype(object)).groups
# Returns {'A': [0, 3], 'B': [1, 2]}
```

### Installed Versions

<details>

INSTALLED VERSIONS
------------------
commit           : 9b2bb732f09d8e870d806b337b97d3d99fd5b118
python           : 3.8.12.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
Version          : 10.0.17134
machine          : AMD64
processor        : Intel64 Family 6 Model 94 Stepping 3, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : English_United States.1252

pandas           : 1.4.0.dev0+952.g9b2bb732f0
numpy            : 1.21.2
pytz             : 2021.1
dateutil         : 2.8.2
pip              : 21.2.4
setuptools       : 58.0.4
Cython           : 0.29.24
pytest           : 6.2.5
hypothesis       : 6.23.1
sphinx           : 4.2.0
blosc            : None
feather          : None
xlsxwriter       : 3.0.1
lxml.etree       : 4.6.3
html5lib         : 1.1
pymysql          : None
psycopg2         : None
jinja2           : 3.0.1
IPython          : 7.28.0
pandas_datareader: None
bs4              : 4.10.0
bottleneck       : 1.3.2
fsspec           : 2021.10.0
fastparquet      : 0.7.1
gcsfs            : 2021.10.0
matplotlib       : 3.4.3
numexpr          : 2.7.3
odfpy            : None
openpyxl         : 3.0.9
pandas_gbq       : None
pyarrow          : 5.0.0
pyxlsb           : None
s3fs             : 2021.10.0
scipy            : 1.7.1
sqlalchemy       : 1.4.25
tables           : 3.6.1
tabulate         : 0.8.9
xarray           : 0.18.2
xlrd             : 2.0.1
xlwt             : 1.3.0
numba            : 0.53.1

</details>
