Move most xfails to entries in tank/all_models.csv and temporarily remove multiprocessing and TF gpu support. (#646)

-Adds date variable back to nightly.yml so shark_tank uploads are dated again
-added specification for nightly pytests to not run tests on metal (vulkan is sufficient)
-added some paths/filetypes to be ignored when triggering workflow runs. (no test-models on changes to .md files or anything in the shark/examples/ directory or its subdirectories.
-pytest only picks up tank/test_models.py, so no need to specify which file to run when running pytest from SHARK base directory.
-Cleaned up xfails so that they can be added to models as csv entries. Columns 7-9 in all_models.csv trigger xfails with cpu, cuda, vulkan, respectively, and row 10 can be populated with a reason for the xfails.
-Fixed a few defaults for shark_args and pytest args (defined in conftest.py)
-Fixes --update_tank option in shark_downloader
removes some multiprocessing in pytest / TF+CUDA support because it breaks pytest and false passes, leaving regressions at large.
-Adds xfails for and removes albert torch from gen_sharktank list (tank/torch_model_list.csv).
-Cleans up xfails for cpu, cuda, vulkan (removing old ones)

pytest.ini

@@ -1,3 +1,3 @@
 [pytest]
 addopts = --verbose -p no:warnings
-norecursedirs = inference tank/tflite 
+norecursedirs = inference tank/tflite examples benchmarks shark