[REVIEW] Fix Slicing issue with categorical column in DataFrame

[rgsl888prabhu]

call to codes materializes the column and gets a new column without any offset which resulted in the issue #4678

closes #4678

[codecov]

Codecov Report

Merging #4683 into branch-0.13 will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@               Coverage Diff               @@
##           branch-0.13    #4683      +/-   ##
===============================================
- Coverage        88.43%   88.42%   -0.01%     
===============================================
  Files               50       50              
  Lines             9676     9696      +20     
===============================================
+ Hits              8557     8574      +17     
- Misses            1119     1122       +3     

Impacted Files	Coverage Δ
python/cudf/cudf/core/dataframe.py	89.12% <ø> (ø)
python/cudf/cudf/core/column/categorical.py	93.67% <100.00%> (+0.12%)	⬆️
python/cudf/cudf/core/column/column.py	87.28% <100.00%> (-0.11%)	⬇️
python/cudf/cudf/core/frame.py	89.32% <100.00%> (+0.03%)	⬆️
python/cudf/cudf/core/reshape.py	86.50% <100.00%> (ø)
python/cudf/cudf/comm/serialize.py	0.00% <0.00%> (ø)
python/dask_cudf/dask_cudf/backends.py	97.14% <0.00%> (ø)
python/cudf/cudf/core/groupby/groupby.py	92.98% <0.00%> (ø)

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