[Fix] dynamic_scatter 'invalid configuration argument error' triggered by empty point input

[zhanggefan]

• fix 'invalid configuration argument' error triggered by empty point input. test cases covering similar situations are added to test_dynamic_scatter.py as well.
  trivial changes:
• switch to using torch::unique_dim to generate reduce mapping instead of calculating it from scratch.

[codecov]

Codecov Report

Merging #417 (e21779c) into master (9cb75e7) will not change coverage.
The diff coverage is n/a.

❗ Current head e21779c differs from pull request most recent head 7f3187e. Consider uploading reports for the commit 7f3187e to get more accurate results

@@           Coverage Diff           @@
##           master     #417   +/-   ##
=======================================
  Coverage   50.70%   50.70%           
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  Files         182      182           
  Lines       12765    12765           
  Branches     2014     2014           
=======================================
  Hits         6473     6473           
  Misses       5875     5875           
  Partials      417      417           

Flag	Coverage Δ
unittests	50.70% <ø> (ø)

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