fix(ltx2): pass actual sequence length to calculate_shift for dynamic timestep shift

[AloysJehwin]

What does this PR do?

Fixes #14243

In LTX2Pipeline.__call__, mu was computed by passing max_image_seq_len as both the first (image_seq_len) and third (max_image_seq_len) arguments to calculate_shift. Since the function returns image_seq_len * m + b, passing the maximum always yields max_shift — so use_dynamic_shifting=True had no effect.

The fix was already suggested in the code: a commented-out line # video_sequence_length = latent_num_frames * latent_height * latent_width sat directly above. Uncommented it and passed the result as image_seq_len, matching LTX v1 pipeline (pipeline_ltx.py:725-728) and the LTX reference implementation.

Before

mu = calculate_shift(
    self.scheduler.config.get(max_image_seq_len, 4096),  # always max -> mu is always max_shift
    ...
    self.scheduler.config.get(max_image_seq_len, 4096),
    ...
)

After

video_sequence_length = latent_num_frames * latent_height * latent_width
mu = calculate_shift(
    video_sequence_length,  # actual sequence length -> mu scales with resolution/frames
    ...
    self.scheduler.config.get(max_image_seq_len, 4096),
    ...
)

Implemented with Claude Code assistance. All changed lines reviewed manually.