[Fix] Allow default TIS function usage when get_mismatch_metrics is set

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Description

This PR addresses a logical conflict between slime/utils/arguments.py and slime/backends/megatron_utils/loss.py.

Motivation

Previously, slime_validate_args enforced a strict assertion that custom_tis_function_path must be provided when get_mismatch_metrics is enabled. However, the backend logic in loss.py explicitly supports a fallback to vanilla_tis_function when no custom path is provided.

Since vanilla_tis_function correctly computes and returns metrics (e.g., tis, tis_clipfrac), the previous assertion unnecessarily prevented users from observing mismatch metrics using the default implementation.

Modification

• Replaced the strict assertion in slime/utils/arguments.py with a warning.
• Now, if get_mismatch_metrics is set without a custom path, the system warns the user (for visibility) and correctly falls back to the default TIS function instead of raising an error.