Numba full backend support and required dependency

[ricardoV94]

What is broken / not supported by Numba:

• float16: meta issue: fp16 support numba/numba#4402
• Sparse operations (progress in Implement Sparse Ops in Numba #1676)
• Non array/scalar global constants Reconsider handlig of constants in numba backend #1759

The plan now

• Numba becomes a default dependency
• Like the default "Mode", "FAST_RUN" is now also an abstract mode that can use either C or Numba, depending on config.linker
  • For now "config.linker" will remain C (specifically CVM) but we'll switch to Numba soon.
  • Why Numba over C?
    • Faster
    • Easier to maintain/ extend
    • Easier to install (pip install should be safe
  • This way downstream libraries can test if anything would break with the switch, and user can switch back if it breaks stuff for them
  • CI will run with both configs. To avoid runtime explosion it may be time to restrict floatX and fast_compile tests to a few files instead of being an orthogonal parametrization like it is now
    • Closes Give test matrix readable names #1755

Immediate action

• I'll be cherry picking the countless fixes into their own PRs. I need help with review as both me and @jessegrabowski are pretty much at capacity Done
• This PR will end up with just the test tweaks and new config flags
• We need to implement more Ops in numba mode. We'll follow up with a PR that fails on fallback to obj mode. Help needed!

[ricardoV94]

Locally I ran all the tests in tests/tensor/rewriting/test_basic.py twice (to allow caching) before and after making Numba default.
Before: 6s
After: 1m34s

Numba caching is baaaaaaaaaaaaaaaaaaaaaaaaaaaaaaad

[ricardoV94]

After caching the test file now runs in 50s after caching vs 6s before the PR, so only 8x slower now :(

[ricardoV94]

The build test is failing with llvm/numba, seems to be trying to build wheel for free threaded python? Do we have a way to skip that, if that's even the problem.

llvmlite/numba don't yet support it (and probably neither do I even if we built for it)

[maresb]

@ricardoV94, just pushed a commit that hopefully solves it

[ricardoV94]

@maresb / @lucianopaz I requested your review basically for the last 2 commits, where we make numba default dependency and run it on the CI

The prior commits is just tweaking tests so they pass with both backends. I've gone over them with @jessegrabowski and asked him to review those. Feel free to look if you want but nothing terribly interesting or critical there

[ricardoV94]

@maresb I force-pushed above your commit, sorry didn't see you had come to the rescue already.

My changes above yours:

should we go with cp*t-*? Mine is too myopic on the python version, and yours is harder to read?
I'll reintroduce your comment.
The test-skip changes I did is because it seemed outdated or redundant from the skip entries. WDYT?