[Fix]: fix compilation error in pytorch 1.7

[ZwwWayne]

In Pytorch 1.7, the cuda kernel of dynamic voxelization will cause compilation errors of error: class "at::Tensor" has no member "max_values" as mentioned in #362. This PR changes to use tensor.max() to fix the error.

Resolves #362

[codecov]

Codecov Report

Merging #393 (1795837) into master (dabf0a2) will not change coverage.
The diff coverage is n/a.

❗ Current head 1795837 differs from pull request most recent head e58419e. Consider uploading reports for the commit e58419e to get more accurate results

@@           Coverage Diff           @@
##           master     #393   +/-   ##
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  Coverage   49.71%   49.71%           
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  Files         181      181           
  Lines       12451    12451           
  Branches     1951     1951           
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  Hits         6190     6190           
  Misses       5871     5871           
  Partials      390      390           

Flag	Coverage Δ
unittests	49.71% <ø> (ø)

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