Fix _is_packed_sequence for 1D and 3D position_ids
Author: winglian
Pull request: yes

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upstreaming fixes from https://github.com/axolotl-ai-cloud/axolotl/pull/3165 and https://github.com/axolotl-ai-cloud/axolotl/pull/3442

_is_packed_sequence assumed position_ids is always [batch_size, seq_len]. Pixtral's position_ids_in_meshgrid can produce a 1D [seq_len] tensor, raising IndexError. Qwen-VL-style MRoPE models pass a 3D [num_position_axes, batch_size, seq_len] tensor, which was misclassified as packed (shape[1] picked up the axis dim instead of seq_len), sending real unpacked batches down the varlen flash-attention path and causing CUDA errors at train time.

Normalize position_ids to 2D before the existing check: unsqueeze a leading batch dim for 1D (only valid for batch_size == 1), and take a single axis for 3D MRoPE inputs, since all axes share the same packing layout.

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cc @arthurzucker for FA2!