Introduce build_conformer_conv() and build_sdpa() in wav2vec2 builder.

[kauterry]

What does this PR do? Please describe:

Refactor wav2vec2 builder to have build_conformer_conv() and build_sdpa() methods which can be overriden in the subclasses.

Hence we can remove causal_depthwise_conv, conv_norm_type, shaw_rel_pos_sdpa_config attributes out of the wav2vec2 config into the subclass' config.

Does your PR introduce any breaking changes? If yes, please list them:
List of all backwards-incompatible changes.

Check list:

• Was the content of this PR discussed and approved via a GitHub issue? (no need for typos or documentation improvements)
• Did you read the contributor guideline?
• Did you make sure that your PR does only one thing instead of bundling different changes together?
• Did you make sure to update the documentation with your changes? (if necessary)
• Did you write any new necessary tests?
• Did you verify new and existing tests pass locally with your changes?
• Did you update the CHANGELOG? (no need for typos, documentation, or minor internal changes)

[cbalioglu]

Just two very nit comments.