Fix missing validation for num_inference_steps in DDPMScheduler and add tests

[Akash504-ai]

What does this PR do?

This PR fixes a missing validation in DDPMScheduler.set_timesteps.

Currently, passing num_inference_steps=0 does not raise an error and can lead to invalid internal state or unexpected behavior.

This behavior is inconsistent with DDIMScheduler, which already validates this case.

Fix

• Added validation in DDPMScheduler.set_timesteps:

  if num_inference_steps is not None and num_inference_steps <= 0:
      raise ValueError("num_inference_steps must be > 0")

Tests

Added new unit test:

• test_num_inference_steps_zero_raises in:
  - test_scheduler_ddpm.py
  - test_scheduler_ddim.py (for consistency)

Result

• Ensures consistent behavior across schedulers
• Prevents silent invalid configurations
• Improves robustness of scheduler API

Fixes #13394

Validation

• All relevant scheduler tests pass locally:
  • pytest tests/schedulers/test_scheduler_ddpm.py
  • pytest tests/schedulers/test_scheduler_ddim.py

@yiyixuxu