BUG: ngroups and len(groups) do not equal when grouping with a list of Grouper and column label
#### Code

```python
m_index = pd.MultiIndex.from_product([['Mona', 'Dennis'], ['good', 'great']], names=['name', 'desc']) 
df = pd.DataFrame({'foo': [1, 2, 1, 2], 'bar': np.random.randn(4)},  index=m_index)
n1 = df.groupby([pd.Grouper(level='name'), 'foo']).ngroups
n2 = len(df.groupby([pd.Grouper(level='name'), 'foo'])) 
n3 = len(df.groupby([pd.Grouper(level='name'), 'foo']).groups) 
print(n1, n2, n3)
# 4 2 2

```
#### Problem description

I grouped a DataFrame by a list of pandas.Grouper object and column label, the length of groups or group keys should be 4, but I checked the length of GroupBy object as n2, it is 2, why? Thanks for your answer in advance.

<details>

ommit: None
python: 3.7.3.final.0
python-bits: 64
OS: Darwin
OS-release: 18.5.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: zh_CN.UTF-8
LOCALE: zh_CN.UTF-8

</details>