[skyrl-train] assert that the policy loss type is regular/dual clip for tis

[erictang000]

TIS is currently only enabled for policy loss types that use the ppo_policy_loss code path

[gemini-code-assist]

Code Review

This pull request adds a validation check to ensure that Truncated Importance Sampling (TIS) is only used with compatible policy loss types, which is a good safeguard. My feedback suggests a small improvement to make this validation more robust by using a ValueError instead of an assert, aligning with the existing validation patterns in the file.

[gemini-code-assist]

For configuration validation, it's better to raise a ValueError instead of using assert. assert statements can be disabled when Python is run with the -O (optimize) flag, which would cause this important validation to be skipped. Using ValueError ensures the check is always performed and is consistent with other checks in this function.

Suggested change

	assert cfg.trainer.algorithm.policy_loss_type in ["regular", "dual_clip"], "TIS is only implemented for regular and dual_clip policy loss types"
	if cfg.trainer.algorithm.policy_loss_type not in ["regular", "dual_clip"]:
	raise ValueError(
	f"TIS is only implemented for 'regular' and 'dual_clip' policy loss types, but got '{cfg.trainer.algorithm.policy_loss_type}'"
	)