Deprecate mae pixio decoder timm (#2006)

* feat: deprecate MAEDecoderTIMM and PixioDecoderTIMM

* refactor: migrate imagenette MAE benchmark to MaskedVisionTransformerDecoderTIMM

* fix: address review on mae/pixio decoder deprecation

- add lightly/utils/deprecation.py::warn_deprecated: FutureWarning (shown by the
  default filter, unlike DeprecationWarning) with the 1.7.0 removal version, called
  from each decoder __init__ so the stacklevel points at the caller
- add a package __getattr__ so importing the deprecated names warns and, once
  removed, raises AttributeError instead of a hintless ImportError
- restore the imagenette benchmark decoder init (apply init_weights to decoder_embed
  and prediction_head) and take sequence_length from the backbone

* fix: warn on decoder subclassing via __init_subclass__ instead of package __getattr__

* fix: guard PixioDecoderTIMM deprecation warning so subclasses warn once

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Co-authored-by: fruet <90271657+gabrielfruet@users.noreply.github.com>