Fix cross-attention cache layer type for T5Gemma2 long inputs

[Beichen-Ma]

Fixes #45521. Cross-attention in T5Gemma2ForConditionalGeneration is supposed to attend to all encoder tokens, but for inputs whose encoder length is >= sliding_window (default 4096) generation crashes with:

RuntimeError: The size of tensor a (4097) must match the size of tensor b (5018) at non-singleton dimension 3

The root cause was in T5Gemma2ForConditionalGeneration._prepare_cache_for_generation, the cross-attention config was being stripped of its sliding-window settings via del:

cross_attn_config = copy.deepcopy(self.config.get_text_config(decoder=True))
del cross_attn_config.sliding_window
del cross_attn_config.layer_types

T5Gemma2DecoderConfig with defaults sliding_window: int | None = 4096 and layer_types: list[str] | None = None. Removing the instance attributes therefore makes attribute lookup fall back to those class defaults, so cross_attn_config once again is sliding_window=4096.

DynamicCache.__init__ sees sliding_window=4096 with layer_types=None will auto-derives layer_types = ["sliding_attention"] * num_hidden_layers, and instantiates DynamicSlidingWindowLayer for every cross-attention layer. On update, those layers truncate the encoder K/V states to the last sliding_window-1 tokens:

self.keys   = full_key_states[:,   :, -self.sliding_window + 1 :, :]
self.values = full_value_states[:, :, -self.sliding_window + 1 :, :]

So when enc_len == 4096, the cached cross-attention keys end up with shape [..., 4095, head_dim], which (after concatenation with the decoder self-attention key in T5Gemma2MergedAttention.forward) yields an attn_weights last-dim of 4097. Hence the mismatch.

Fix

Explicitly set sliding_window to null and layer_types to full attention for all layers, instead of deleting the instance attributes.

Tests

• Added test T5Gemma2ModelTest::test_cross_attention_cache_is_not_sliding, which asserts that after generate() every layer of output.past_key_values.cross_attention_cache is DynamicLayer. Confirmed test fails on main branch and passes on this branch.
• tests/models/t5gemma2/test_modeling_t5gemma2.py passes.
• Verified provided end-to-end reproducer passed after the fix.

python /tmp/transformers_bug_repro.py 
Loading weights: 100%|███████████████████████████████████████████████████████████████████████████████████| 1327/1327 [00:04<00:00, 323.79it/s]

--- target=2500 ---
[transformers] The following generation flags are not valid and may be ignored: ['top_p', 'top_k']. Set `TRANSFORMERS_VERBOSITY=info` for more details.
OK  (input=2500, output=17)

--- target=3500 ---
OK  (input=3500, output=17)

--- target=4000 ---
OK  (input=4000, output=17)

--- target=4090 ---
OK  (input=4090, output=17)

--- target=4100 ---
OK  (input=4100, output=17)

--- target=4500 ---
OK  (input=4500, output=17)

--- target=5000 ---
OK  (input=5000, output=17)

--- target=8000 ---
OK  (input=8000, output=17)

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[github-actions]

[For maintainers] Suggested jobs to run (before merge)

run-slow: t5gemma2

[Rocketknight1]

cc @vasqu since you were active in the original issue!