Refactor assignment logic to aliasing same column multiple times on GPU

[charlesbluca]

Refactors our DataContainer.assign logic to avoid creating a dataframe with multiple columns on GPU, which silently fails and causes errors later down the road.

Closes #1133

[codecov-commenter]

Codecov Report

Merging #1134 (aca4a44) into main (108e024) will decrease coverage by 0.17%.
The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##             main    #1134      +/-   ##
==========================================
- Coverage   81.42%   81.25%   -0.17%     
==========================================
  Files          78       78              
  Lines        4392     4390       -2     
  Branches      796      796              
==========================================
- Hits         3576     3567       -9     
- Misses        639      640       +1     
- Partials      177      183       +6     

Impacted Files	Coverage Δ
dask_sql/datacontainer.py	95.45% <100.00%> (-0.09%)	⬇️

... and 2 files with indirect coverage changes

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