fix: Correct Switch Transformers sparse layer logic and outdated spectrogram comment

[harshaljanjani]

What does this PR do?

The following issues were identified and fixed in this PR:

1. SwitchTransformersConfig incorrectly created sparse (MoE) layers when num_sparse_encoder_layers=0 and num_layers=1 due to prev logic. The modeling code’s sparse_step == 1 condition would then trigger, creating a sparse layer when none were requested. This is fixed by setting encoder_sparse_step = 0 and decoder_sparse_step = 0 when no sparse layers are requested, which the modeling code already handles correctly via if sparse_step > 0 else False.
2. Updated an outdated comment in audio_utils.py that stated spectrogram() does not support batching, even though spectrogram_batch() already exists. Changed to a note referencing the batch function.

Fixes #43335.

Before submitting

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• Did you write any new necessary tests?

cc: @Rocketknight1

[github-actions]

[For maintainers] Suggested jobs to run (before merge)

run-slow: switch_transformers

[github-actions]

View the CircleCI Test Summary for this PR:

https://huggingface.co/spaces/transformers-community/circle-ci-viz?pr=43336&sha=372d5d

[harshaljanjani]

The failing tests are unrelated to this change; would appreciate a review when you get a chance thanks!