refactor sana transformer tests

[akshan-main]

What does this PR do?

Part of the ongoing modeling-test migration (following #13369 and #13153). Refactors the Sana transformer tests into a SanaTransformerTesterConfig plus per-component test classes: ModelTesterMixin, MemoryTesterMixin, TrainingTesterMixin, and AttentionTesterMixin.

SanaTransformer2DModel doesn't set _repeated_blocks, so the compile mixin is not included.

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Who can review?

@sayakpaul