Fix missing raise before NotImplementedError in Flux SPMD example

[Ricardo-M-L]

What does this PR do?

In examples/research_projects/pytorch_xla/inference/flux/flux_inference_spmd.py, the NotImplementedError exception object is created but never raised when num_devices < 4:

# Before (bug):
else:
    NotImplementedError  # creates object, doesn't raise it

# After (fix):
else:
    raise NotImplementedError(f"SPMD mesh requires at least 4 devices, got {num_devices}")

Without raise, execution continues to the next line where mesh is undefined, causing a confusing NameError instead of the intended informative error.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline?

Who can review?

@yiyixuxu @sayakpaul