Minor dask_cudf sort_values optimization for single partitions

[rjzamora]

Closes #3873 (Although we do still want to improve general performance -- In progress)

This PR adds a small change to effectively "short circuit" the usual batcher-sortnet algorithm when there is only a single partition.

I also modified the behavior of ignore_index, because it seemed to be "incorrect" (however, I'm happy to revert this if I'm mistaken)

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (branch-0.13@fc73d7b). Click here to learn what that means.
The diff coverage is 87.5%.

@@              Coverage Diff               @@
##             branch-0.13    #4000   +/-   ##
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  Coverage               ?   86.67%           
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  Files                  ?       50           
  Lines                  ?     9724           
  Branches               ?        0           
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  Hits                   ?     8428           
  Misses                 ?     1296           
  Partials               ?        0

Impacted Files	Coverage Δ
python/dask_cudf/dask_cudf/core.py	68.4% <87.5%> (ø)

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