Fix wrong parameter name in diffusion attention-mask error message

[PratikDhanave]

What

In gemma/diffusion/_sampler.py, _make_global_attention_mask validates its num_valid_tokens argument:

if num_valid_tokens is None:
    raise ValueError(
        'num_valid_samples must be provided if cache_length is set.'
    )

The message refers to num_valid_samples, which is not a parameter of the function — the argument is named num_valid_tokens. Anyone hitting this error would look for a num_valid_samples argument that doesn't exist.

The sibling guard in _make_block_local_attention_mask already uses the correct name ('num_valid_tokens must be provided ...').

Change

Align the error message with the actual parameter name. Message-only; no behavior change.

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