fix override errors in masked causal vision transformer (#1913)

* Fix override type errors in MaskedCausalVisionTransformer

Add # type: ignore[override] to three method signatures:
- MaskedCausalAttention.forward
- MaskedCausalBlock.forward
- MaskedCausalVisionTransformer.forward_features

These classes intentionally use a simplified mask parameter API
while inheriting from timm's classes which use attn_mask, is_causal.
The override error is a false positive—method signatures are
different by design.

* feat: use proper timm's attention signature.

* fix: unecessary import

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Co-authored-by: Igor Susmelj <igor_7779@hotmail.com>