Using DTensor to handle local num_heads change while TP is applied#3465
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🔗 Helpful Links🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/tutorials/3465
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tianyu-l
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| If there are any more tensor operations (such as view operations) between the column-wise linear and the row-wise linear, we would need to adjust the relevant shape related ops to sharded shape. | ||
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| For the Llama model, in the attention layer there are couple of view operations that are shape related. In particular, column-wise parallel for ``wq``/ ``wk``/ ``wv`` linear layers, the activation tensor is sharded on the ``num_heads`` dimension, so we would need to adjust the ``num_heads`` to local ``num_heads``. | ||
| For the Llama model, in the attention layer, there are several view operations related to shape. Specifically, for column-wise parallelism in the ``wq``/``wk``/``wv`` linear layers, the activation tensor is sharded on the ``num_heads`` dimension. To manage the difference between global and local ``num_heads``, we should set ``use_local_output=False`` to ensure the output is a DTensor. Unlike a regular tensor, a DTensor is aware of the parallelism plans and will automatically handle changes in the ``num_heads`` dimension. |
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I think we should be able to use DTensor i.e. set use_local_output=False everywhere.
Maybe it's OK to keep a mixed usage of use_local_output so people are aware of this flexibility, but we should mention it here.
svekars
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Jul 23, 2025
…3465) * fsdp1 -> fsdp2 * change num_heads in tutorial --------- Co-authored-by: Svetlana Karslioglu <svekars@meta.com>
mikaylagawarecki
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…3465) * fsdp1 -> fsdp2 * change num_heads in tutorial --------- Co-authored-by: Svetlana Karslioglu <svekars@meta.com>
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Fixes #ISSUE_NUMBER. This PR is to make the TP tutorial up-to-date with DTensor changes.
Description
After DTensor enhancement, we are not able to use DTensor to handle the change of
num_headsinstead of manually handle the tensor shape while TP is applied.Corresponding changes in
pytorch/examples: pytorch/examples#1373Checklist