added mean support for ints on local level, also a correctness test

[coquelin77]

Description

adds support for ints to mean and variance. The problem was at the pytorch level, the solution was to cast the array to a float. all tests run without failure

Fixes: #349 (and parallel problem with var although it was not previously pointed out)

Changes proposed:

• casting of the torch tensor to float in the non-distributed case for both mean and var (has no effect if the tensor is already a float)

Type of change

• Bug fix (non-breaking change which fixes an issue)

Have you handled and tested all split configurations?
Yes

[codecov-io]

Codecov Report

Merging #350 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #350      +/-   ##
==========================================
+ Coverage   96.95%   96.96%   +<.01%     
==========================================
  Files          53       53              
  Lines        8913     8917       +4     
==========================================
+ Hits         8642     8646       +4     
  Misses        271      271

Impacted Files	Coverage Δ
heat/core/tests/test_statistics.py	100% <100%> (ø)	⬆️
heat/core/statistics.py	96.53% <100%> (ø)	⬆️

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[ClaudiaComito]

Great, thanks!