[v0.5.10][7][FSDP] move FSDP to experimental and disable by default

[yueming-yuan]

ci-sglang-pr: sglang-miles-v0.5.10

Summary

• Move miles/backends/fsdp_utils/ to miles/backends/experimental/fsdp_utils/ and update all import paths
• Add assertion in argument validation to block --train-backend fsdp with deprecation message
• Remove FSDP tests from CI jobs except run-ci-fsdp and run-ci-image
• Add [FSDP] prefix to all FSDP test names in CI matrix

[gemini-code-assist]

Code Review

This pull request moves the FSDP backend to the experimental directory and adds a runtime check to warn users about its current maintenance status. The reviewer correctly identified that using an assertion for this validation is unsafe because it can be disabled with the -O flag, and suggested replacing it with a ValueError to ensure the check is always enforced.

[gemini-code-assist]

Using assert for runtime validation of user-provided arguments is generally discouraged in Python. Assertions can be globally disabled using the -O optimization flag, which would bypass this safety check. For critical validation intended to block a feature or provide a user-facing error message, it is better to raise an exception like ValueError to ensure the check is always performed.

Suggested change

	assert backend != "fsdp", (
	"The FSDP backend has known issues with SGLang v0.5.10 and is not actively maintained in the current version. "
	"It has been moved to miles.backends.experimental. "
	"Contributions are welcome if you are interested in improving it."
	)
	if backend == "fsdp":
	raise ValueError(
	"The FSDP backend has known issues with SGLang v0.5.10 and is not actively maintained in the current version. "
	"It has been moved to miles.backends.experimental. "
	"Contributions are welcome if you are interested in improving it."
	)

[maocheng23]

Merge after resolve conflict please