Adding Mac to S3 non-GPL FFmpeg compile

[scotts]

Adds CI workflows so that we build from S3 on Mac arm64 (the architecture for M1 processors). Enabling Mac arm64 builds required changes in several places:

1. Modified fetch_and_expose_non_gpl_ffmpeg_libs.cmake to handle Mac OS on arm64. I had to generalize the logic so it can handle both Mac and Linux. This includes pulling the right tar files from S3 and specifying the right library names. (Linux uses .so; Mac uses .dylib.)
2. Added .github/workflows/macos_wheel.yaml. The goal for this PR is to get it to the state where it builds from the S3 binaries and runs the Python tests. Subsequent PRs will turn this into the actual wheel building and testing.
3. Renamed .github/workflows/wheel.yaml to linux_wheel.yaml.
4. Modified test/utils.py so that when testing on Macs, we allow an absolute tolerance of 3 when comparing tensors. This means that we are okay with not having bit-for-bit-equality between Mac and Linux. Based on what I can find, I think trying to maintain bit-for-bit equality between systems and architectures is unrealistic. We will maintain bit-for-bit equality on Linux x86.
5. Modified VideoDecoderOps.cpp to throw a pybind11 exception into one specific to PyTorch. The pybind11 exception was supposed to become a StopIteration exception in Python. That worked on Linux, but it segfaulted on Mac. PyTorch does not have an equivalent of StopIteration, but it does have one that becomes IndexError. I think that's close enough. From what I can tell, PyTorch does not always play nice with pybind11 exceptions; see END_HANDLE_TH_ERRORS_PYBIND prevents pybind11 Exception translation pytorch#34172. I opened Issue C++ code should not throw its own exceptions #212 to track using PyTorch errors everywhere in the C++ code.