ENH: len for groupby

[RayJi01]

What do these changes do?

Inplement len Operands for groupby Object. e.g. calling len(grouped_object)

Related issue number

Fixes #442

Check code requirements

• tests added / passed (if needed)
• Ensure all linting tests pass

[codecov]

Codecov Report

Merging #533 (0cf151b) into main (895af3b) will increase coverage by 3.87%.
The diff coverage is 98.38%.

@@            Coverage Diff             @@
##             main     #533      +/-   ##
==========================================
+ Coverage   89.80%   93.67%   +3.87%     
==========================================
  Files        1008     1009       +1     
  Lines       77917    77979      +62     
  Branches    16170    16179       +9     
==========================================
+ Hits        69971    73046    +3075     
+ Misses       6143     3298    -2845     
+ Partials     1803     1635     -168     

Flag	Coverage Δ
unittests	93.56% <98.38%> (+3.87%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
python/xorbits/_mars/dataframe/groupby/len.py	98.27% <98.27%> (ø)
python/xorbits/_mars/dataframe/groupby/__init__.py	96.55% <100.00%> (+0.12%)	⬆️
python/xorbits/_mars/opcodes.py	100.00% <100.00%> (ø)

... and 128 files with indirect coverage changes