Fix DreamLite legacy block type aliases

[ElectricGoal]

What does this PR do?

Fixes #14065.

carlofkl/DreamLite-base saves DreamLite UNet block type names using upstream-style strings such as CrossAttnDownRemoveSelfAttnBlock2D, CrossAttnUpBlock2D, and CrossAttnUpRemoveSelfAttnBlock2DV1.

DreamLiteUNetModel currently only accepts the DreamLite-local class names, so DreamLitePipeline.from_pretrained("carlofkl/DreamLite-base") fails while constructing the UNet.

This PR adds a small alias normalization step in the DreamLite block dispatch helpers so those saved config values map to the existing DreamLite block classes. It also adds a focused regression test using the exact block strings from the public
checkpoint config.

Coordination issue: #14065

Before submitting

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  • Did you read the Coding with AI agents guide?
  • Did you self-review the diff against .ai/review-rules.md?
• Did you read the contributor guideline?
• Did you read our philosophy doc? (important for complex PRs)
• Was this discussed/approved via a GitHub issue or the forum? DreamLite-base fails to load due to unsupported legacy UNet block type names #14065
• Did you make sure to update the documentation with your changes? No docs update needed; this only restores compatibility with existing saved DreamLite configs.
• Did you write any new necessary tests?
• Are you the author (or part of the team) of the model/pipeline (only applicable for model/pipeline related PRs)?

Tests

../.venv_dreamlite/bin/python -m pytest tests/pipelines/dreamlite/test_pipeline_dreamlite.py::DreamLitePipelineFastTests::test_legacy_block_type_aliases -q
# 1 passed, 3 warnings in 1.36s

../.venv_dreamlite/bin/python -m ruff check src/diffusers/models/unets/unet_dreamlite.py tests/pipelines/dreamlite/test_pipeline_dreamlite.py
# All checks passed!

../.venv_dreamlite/bin/python -m ruff format --check src/diffusers/models/unets/unet_dreamlite.py tests/pipelines/dreamlite/test_pipeline_dreamlite.py
# 2 files already formatted

../.venv_dreamlite/bin/python -m compileall -q src/diffusers/models/unets/unet_dreamlite.py tests/pipelines/dreamlite/test_pipeline_dreamlite.py
git diff --check
# passed

Who can review?

Pipelines and models: @Carlofkl