Typehints for model_loader and model_service_worker

[ZachOBrien]

Description

1. Adds necessary typehinting and checks for None to make mypy happy in two files: ts/model_loader.py, ts/model_service_worker.py
2. Adds unit test for None check in handle_connection

Related to #1512

Type of change

Code quality - typehints

Feature/Issue validation/testing

• Added unit test for new None checking in handle_connection.
  See ts/tests/unit_tests/test_model_service_worker.py
• Ran mypy on changed files
• Ran sanity check (torchserve_sanity.py)
• Ran regression tests (test/regression_tests.py)

Checklist:

• Did you have fun?
• Have you added tests that prove your fix is effective or that this feature works?
• Has code been commented, particularly in hard-to-understand areas?
• Have you made corresponding changes to the documentation?

[msaroufim]

lgtm, let's see what ci says

[codecov]

Codecov Report

Merging #2540 (4d43b5a) into master (fc814be) will increase coverage by 0.06%.
The diff coverage is 81.25%.

❗ Current head 4d43b5a differs from pull request most recent head 866bf0d. Consider uploading reports for the commit 866bf0d to get more accurate results

@@            Coverage Diff             @@
##           master    #2540      +/-   ##
==========================================
+ Coverage   72.64%   72.71%   +0.06%     
==========================================
  Files          79       79              
  Lines        3733     3742       +9     
  Branches       58       58              
==========================================
+ Hits         2712     2721       +9     
  Misses       1017     1017              
  Partials        4        4              

Files Changed	Coverage Δ
ts/model_loader.py	88.31% <ø> (ø)
ts/model_service_worker.py	64.38% <66.66%> (+0.49%)	⬆️
ts/tests/unit_tests/test_model_service_worker.py	99.18% <100.00%> (+0.04%)	⬆️

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[ZachOBrien]

Hm, I haven't been able to replicate the ci regression test errors locally. It seems odd that in the first run only the GPU regression test failed, but after merging in master the GPU regression test passed but the CPU and macOS failed.

[msaroufim]

Regressions I believe are unrelated to you so we can still merge this