Add Pyright

[brentyi]

I added Pyright and started fixing type errors.

On main:

~/mjlab uv run pyright
[...]
434 errors, 0 warnings, 0 informations

On this branch:

~/mjlab uv run pyright
0 errors, 0 warnings, 0 informations

It seems like most of these were real issues. I tried my best to fix, but there were a few things I wasn't sure about:

• The correct type for indexing when resetting. We sometimes had torch.Tensor | slice | None in the type signature, but didn't actually support all of them at runtime. Should we standardize?
• The .func attribute. This was generally annotated as func: Callable, but we sometimes also call .func.reset(env_ids).
• Incompatible overrides of gym.Env, for example returning tensors instead of ints. I see why this makes sense, I guess the only solutions is type: ignore?

[kevinzakka]

Nice, pyright passes for me too now!