[Build] Refactor backends to adhere to stricter build warnings

[VivekPanyam]

Summary:

Refactor code under source/neuropod/backends to adhere to the stricter set of warnings introduced in #437

Test Plan:

CI

[codecov]

Codecov Report

Merging #442 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #442   +/-   ##
=======================================
  Coverage   88.23%   88.23%           
=======================================
  Files         101      101           
  Lines        6356     6356           
=======================================
  Hits         5608     5608           
  Misses        748      748           

Impacted Files	Coverage Δ
source/neuropod/internal/backend_registration.hh	100.00% <ø> (ø)
source/neuropod/backends/neuropod_backend.cc	96.48% <100.00%> (ø)
source/neuropod/backends/tensorflow/tf_backend.cc	89.12% <100.00%> (ø)
source/neuropod/backends/tensorflow/tf_tensor.cc	63.63% <100.00%> (ø)
source/neuropod/backends/tensorflow/tf_tensor.hh	86.56% <100.00%> (ø)
...rce/neuropod/backends/torchscript/torch_backend.cc	81.47% <100.00%> (ø)
...urce/neuropod/backends/torchscript/torch_tensor.hh	74.19% <100.00%> (ø)

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[vkuzmin-uber]

you could make it "inline", this is legal way to provide definition in header w/o confusing compiler. But I see you added .cc file that is also fine.

[VivekPanyam]

Yup! Previously, I marked it as inline in an internal patch on top of Neuropod, but decided to pull it into a .cc file here.

I was planning on pulling more things into the .cc file, but taking another look, it probably makes sense to just inline this in the header and leave it alone

[vkuzmin-uber]

LGTM