Fix FlaxLMS step timestep→index; complete get_lms_coefficient docs

[HaoyuLi-Nova]

What does this PR do?

This PR fixes FlaxLMSDiscreteScheduler.step so it maps the timestep value passed from Flax pipelines (the label
stored in state.timesteps[i]) to a step index via jnp.where, consistent with FlaxEulerDiscreteScheduler and
with PyTorch LMSDiscreteScheduler (which uses step_index). It also adds prediction_type="sample" for parity
with the PyTorch scheduler, and completes get_lms_coefficient docstrings/types (including explicit float return).

No new runtime dependencies.

N/A (no tracking issue)

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Who can review?

@yiyixuxu