BUG: Multiindex.nunique raises NotImplementedError
- [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.

---

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

```python
pd.DataFrame([[1,2],[1,2]]).set_index([0,1]).index.nunique()
```
```python
---------------------------------------------------------------------------
NotImplementedError                       Traceback (most recent call last)
<ipython-input-80-09716b89b157> in <module>
----> 1 pd.DataFrame([[1,2],[1,2]]).set_index([0,1]).index.nunique()

/opt/conda/lib/python3.7/site-packages/pandas/core/base.py in nunique(self, dropna)
   1284         uniqs = self.unique()
   1285         n = len(uniqs)
-> 1286         if dropna and isna(uniqs).any():
   1287             n -= 1
   1288         return n

/opt/conda/lib/python3.7/site-packages/pandas/core/dtypes/missing.py in isna(obj)
    124     Name: 1, dtype: bool
    125     """
--> 126     return _isna(obj)
    127 
    128 

/opt/conda/lib/python3.7/site-packages/pandas/core/dtypes/missing.py in _isna_new(obj)
    136     # hack (for now) because MI registers as ndarray
    137     elif isinstance(obj, ABCMultiIndex):
--> 138         raise NotImplementedError("isna is not defined for MultiIndex")
    139     elif isinstance(obj, type):
    140         return False

NotImplementedError: isna is not defined for MultiIndex

```
exactly same error raised by pandas 0.25.3

#### Problem description

The method exists, but always throws NotImplementedError. I can use len(df.index.unique()) as a substitute. In the previous version of pandas, documentation have Multiindex.nunique entry (https://pandas.pydata.org/pandas-docs/version/0.23.4/generated/pandas.MultiIndex.unique.html), but it has disappeared now. So I think it might be possible that this is not a bug, but an intended exception.

#### Expected Output

should be 1

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

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.6.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.19.76-linuxkit
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : en_US.UTF-8
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.0.3
numpy            : 1.18.1
pytz             : 2020.1
dateutil         : 2.8.1
pip              : 20.1
setuptools       : 46.1.3.post20200325
Cython           : 0.29.17
pytest           : 5.4.1
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : 1.2.8
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.2
IPython          : 7.14.0
pandas_datareader: None
bs4              : 4.9.0
bottleneck       : 1.3.2
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : 3.2.1
numexpr          : 2.7.1
odfpy            : None
openpyxl         : 3.0.3
pandas_gbq       : None
pyarrow          : 0.17.0
pytables         : None
pytest           : 5.4.1
pyxlsb           : None
s3fs             : None
scipy            : 1.4.1
sqlalchemy       : 1.3.16
tables           : 3.6.1
tabulate         : None
xarray           : 0.15.1
xlrd             : 1.2.0
xlwt             : None
xlsxwriter       : 1.2.8
numba            : 0.48.0
</details>
