BUG: Lacking milliseconds field in DateOffset

[isVoid]

• I have checked that this issue has not already been reported.

• 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.

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Code Sample, a copy-pastable example

# Your code here
>>> import pandas as pd
>>> pd.date_range(start='2000-01-01 08:00:00', end='2000-01-01 08:00:01', freq='1L')
DatetimeIndex([       '2000-01-01 08:00:00', '2000-01-01 08:00:00.001000',
               '2000-01-01 08:00:00.002000', '2000-01-01 08:00:00.003000',
               '2000-01-01 08:00:00.004000', '2000-01-01 08:00:00.005000',
               '2000-01-01 08:00:00.006000', '2000-01-01 08:00:00.007000',
               '2000-01-01 08:00:00.008000', '2000-01-01 08:00:00.009000',
               ...
               '2000-01-01 08:00:00.991000', '2000-01-01 08:00:00.992000',
               '2000-01-01 08:00:00.993000', '2000-01-01 08:00:00.994000',
               '2000-01-01 08:00:00.995000', '2000-01-01 08:00:00.996000',
               '2000-01-01 08:00:00.997000', '2000-01-01 08:00:00.998000',
               '2000-01-01 08:00:00.999000',        '2000-01-01 08:00:01'],
              dtype='datetime64[ns]', length=1001, freq='L')
>>> pd.date_range(start='2000-01-01 08:00:00', end='2000-01-01 08:00:01', freq=pd.DateOffset(milliseconds=1))
DatetimeIndex(['2000-01-01 08:00:00'], dtype='datetime64[ns]', freq='<DateOffset>')

Problem description

Today, I can specify 1 millisecond offset in date_range by offset alias 1L or 1ms. But I cannot specify this offset through pd.DateOffset because its lacking a milliseconds field.

Expected Output

Similar to the case specified with offset alias L or ms.

Output of pd.show_versions()

INSTALLED VERSIONS

commit : 7c48ff4
python : 3.7.10.final.0
python-bits : 64
OS : Linux
OS-release : 4.15.0-76-generic
Version : #86-Ubuntu SMP Fri Jan 17 17:24:28 UTC 2020
machine : x86_64
processor : x86_64
byteorder : little
LC_ALL : None
LANG : None
LOCALE : en_US.UTF-8

pandas : 1.2.5
numpy : 1.21.2
pytz : 2021.1
dateutil : 2.8.1
pip : 21.2.4
setuptools : 52.0.0.post20210125
Cython : 0.29.24
pytest : 6.2.4
hypothesis : 6.17.2
sphinx : 4.1.2
blosc : None
feather : None
xlsxwriter : None
lxml.etree : None
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : 2.11.3
IPython : 7.27.0
pandas_datareader: None
bs4 : 4.9.3
bottleneck : None
fsspec : 2021.07.0
fastparquet : None
gcsfs : None
matplotlib : 3.4.3
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : 5.0.0
pyxlsb : None
s3fs : None
scipy : 1.5.3
sqlalchemy : None
tables : None
tabulate : None
xarray : None
xlrd : None
xlwt : None

[venaturum]

This seems more like a feature request than a bug. The documentation doesn't list microseconds as being a valid frequency for DateOffset.