fix: Retain continuous approximation of Poisson in torch v1.8.0

[matthewfeickert]

Description

Resolves #1348

To retain the continuous approximation of the Poisson, set validate_args=False in instantiations of the torch.distributions.Poisson class.

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* Add validate_args=False to instantiations of the torch.distributions.Poisson class in PyTorch backend to allow for the continuous approximation of the Poisson distribution in torch v1.8+
* Ensure tensor value arguments for torch.distributions.Normal
* Check that ValueErrors are raised by PyTorch v1.8+ when the validations of validate_args are violated

[matthewfeickert]

@lukasheinrich @kratsg I'm still thinking on this with regards to how we can make this better / cleaner. Though I welcome your thoughts and input now.

[codecov]

Codecov Report

Merging #1351 (3040eac) into master (2c10be3) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1351   +/-   ##
=======================================
  Coverage   97.50%   97.50%           
=======================================
  Files          63       63           
  Lines        3762     3768    +6     
  Branches      538      538           
=======================================
+ Hits         3668     3674    +6     
  Misses         55       55           
  Partials       39       39           

Flag	Coverage Δ
unittests	97.50% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
src/pyhf/tensor/pytorch_backend.py	98.27% <100.00%> (+0.09%)	⬆️

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