Keep `torch` dependency from bloating our common pixi environments

[jleibs]

Even though pixi is fairly efficient when it comes to downloading, caching, etc. the torch dep still adds several gigs to the environment.

This has two notable consequences:

• It actually causes smaller github runners to fail because they run out of storage. (https://github.com/rerun-io/rerun/pull/6964)
• It causes significantly more churn on the github cache of the pixi environments since we blow the 10GB limit with just a few environments.

We currently need torch for 2 things:

• Running unitests that depend on the torch tensor type
• Running a handful of examples that depend on torch

Proposal

• Make the torch dep for unit-tests optional. Only include torch arrays if the dep is present.
  • If we're feeling particularly fancy we could fallback-import a torch tensor shim, but I'm not sure it's worth the effort.
• Split out the torch environment dep so we end up with wheel-test and wheel-test-torch
• Remove any examples that need torch from main
• Only use wheel-test-torch on nightly to:
  • Validate the unittests still pass with torch
  • Run any examples that depend on torch
• Disable saving the wheel-test-torch cache. Since we only run it nightly it's not worth causing cache churn for it.

[emilk]

Maybe things depending on torch should be in its own repo?

• https://github.com/rerun-io/rerun/issues/5661