Fix qwen encoder hidden states mask

[cdutr]

What does this PR do?

Fixes the QwenImage encoder to properly apply encoder_hidden_states_mask when passed to the model. Previously, the mask parameter was accepted but ignored, causing padding tokens to incorrectly influence attention computation.

Changes

• Attention mask application: Modified QwenDoubleStreamAttnProcessor2_0 to create a 2D attention mask from the 1D encoder_hidden_states_mask, properly masking text padding tokens while keeping all image tokens unmasked
• RoPE adjustment: Updated positional embedding computation to use the full padded sequence length when a mask is present, ensuring correct position indices
• Tests: Added comprehensive tests validating that:
  • Padding tokens are properly isolated and don't affect outputs
  • Masked outputs differ significantly from unmasked outputs
• Benchmarks: Included performance analysis showing acceptable overhead (<20% for inference, ~19% for training scenarios)

Impact

This fix enables proper Classifier-Free Guidance (CFG) batching with variable-length text sequences, which is common when batching conditional and unconditional prompts together.

Benchmark Results

Scenario	Latency (ms)	Peak Memory (MB)	Throughput (iter/s)
Baseline (no mask)	11.68 ± 0.23	301.5	84.70
Mask all-ones (no padding)	12.01 ± 0.26	301.5	82.34
Mask with padding (CFG)	13.86 ± 0.24	301.5	71.42

Overhead: +2.8% for mask processing without padding, +18.7% with actual padding (realistic CFG scenario)

The higher overhead with padding is expected and acceptable as it represents the cost of properly handling variable-length sequences in batched inference. This is a necessary correctness fix rather than an optimization. Test ran on RTX 4070 12GB.

Fixes #12294

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Before submitting

• This PR fixes a bug in the code
• This PR adds tests that verify the fix
• This PR includes benchmarks demonstrating performance impact
• Did you write any new necessary tests?

Who can review?

@yiyixuxu @sayakpaul - Would appreciate your review, especially regarding the benchmarking approach. I used a custom benchmark rather than BenchmarkMixin because:

1. This tests a specific bug fix (mask application) rather than optimization strategies
2. The fix uses synthetic models to isolate the mask handling logic
3. Standard benchmarks focus on pretrained model performance with different quantization/offloading strategies
4. The metrics needed are different (latency distribution, throughput) vs standard format (compile/plain time comparison)

Note: The benchmark file is named benchmarking_qwenimage_mask.py (with "benchmarking" prefix) rather than benchmark_qwenimage_mask.py to prevent it from being picked up by run_all.py, since it doesn't use BenchmarkMixin and produces a different CSV schema. If you prefer, I can adapt it to use the standard format instead.

Happy to adjust the approach if you have suggestions!