Remove 'partition_by' requirement from 'group_by' method

[dreadatour]

This will allow us to run queries like this:

from datachain import C, DataChain, func
from sqlalchemy import case


(
  DataChain.from_values(name=["foo", "bar", "baz"], anxiety=[0.0, 1.0, 0.5], boredom=[0.7, 0.0, 0.05])
  .mutate(
    anxiety=case((C("anxiety") > 0.1, 1), else_=0),
    boredom=case((C("boredom") > 0.1, 1), else_=0),
  )
  .group_by(
    anxiety=func.sum("anxiety"),
    boredom=func.sum("boredom"),
  )
  .show()
)

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[dreadatour]

Will add tests for group_by without partition_by in the follow-up PR.

[codecov]

Codecov Report

Attention: Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 87.64%. Comparing base (bf7d670) to head (84f702c).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/datachain/lib/dc.py	33.33%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #649      +/-   ##
==========================================
+ Coverage   87.62%   87.64%   +0.02%     
==========================================
  Files         111      111              
  Lines       10599    10598       -1     
  Branches     1436     1435       -1     
==========================================
+ Hits         9287     9289       +2     
+ Misses        948      946       -2     
+ Partials      364      363       -1     

Flag	Coverage Δ
datachain	87.59% <60.00%> (+0.02%)	⬆️

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