Fixing casts to int by to_torch_as(...) calls in policies when using discrete actions

[Kenneth-Schroeder]

• I have marked all applicable categories:
  • exception-raising fix
  • algorithm implementation fix
  • documentation modification
  • new feature
• I have reformatted the code using make format (required)
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related issue: #520

[codecov-commenter]

Codecov Report

Merging #521 (5ff3145) into master (c25926d) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #521   +/-   ##
=======================================
  Coverage   94.41%   94.41%           
=======================================
  Files          63       63           
  Lines        4246     4246           
=======================================
  Hits         4009     4009           
  Misses        237      237           

Flag	Coverage Δ
unittests	94.41% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
tianshou/policy/modelfree/pg.py	98.21% <100.00%> (ø)

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[Trinkle23897]

LGTM. Double checked and only here has the issue.