[modular] Tests for custom blocks in modular diffusers

[sayakpaul]

What does this PR do?

As the title suggests, this PR adds a test suite for the custom blocks in modular diffusers.

• TestModularCustomBlocks tests for basic checks, e.g., properties, configs, components, inputs, etc. It also has a test that checks for Hub loading and its correctness.
• TestKreaCustomBlocksIntegration checks for correct loading and also the forward pass to ensure nothing is broken as we continue to improve modular diffusers.

Notes:

• I took the liberty to simplify the .github/workflows/pr_modular_tests.yml, getting rid of the matrix as it wasn't really needed.
• The tests were implemented in pytest because we want to slowly move to it from unittest. pytest makes things easier to maintain long term.
• The TestKreaCustomBlocksIntegration test suite was checked in an H100 because it is a bit heavy.

[HuggingFaceDocBuilderDev]

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