[draft][compile][graph_partition]Add tensor size handling

[fxdawnn]

Purpose

Fix #31043

Summary

split_graph partitions an FX graph into subgraphs at splitting ops (e.g., attention, sigmoid). On main, sym_size.int nodes (from tensor.shape[dim]) can end up in consumer subgraphs after a split boundary. This causes the original tensor to be passed as an input to the consumer just for .size() calls, keeping it alive unnecessarily.

This PR adds a pre-pass that moves sym_size.int nodes to right after their tensor operand before subgraph assignment. The normal sequential assignment then places them in the producer subgraph. split_module threads the SymInt result to consumers automatically.

main (current):

submod_0: (empty or other ops)
submod_1: sigmoid(x)                                              # split point
submod_2: sym_size.int(x, 0), sym_size.int(x, 1), view(y, ...)   # x passed for .size()

This PR:

submod_0: sym_size.int(x, 0), sym_size.int(x, 1)   # shape computed in producer
submod_1: sigmoid(x)                                 # split point
submod_2: view(y, s0, s1)                            # only s0, s1 (SymInt) cross boundary

Why this works

split_module already threads SymInt values across subgraph boundaries correctly. Once sym_size.int computes the result in the producer, split_module passes that SymInt to any consumer that needs it — no custom SymInt handling code required.

This follows the same pattern as the existing getitem hoisting (PR #28533), which moves getitem nodes to the producer to avoid passing tuples across boundaries.

Change

One pre-pass added to split_graph (~10 lines):

for node in list(graph.graph.nodes):
    if (node.op == "call_function"
            and node.target == torch.ops.aten.sym_size.int):
        tensor_node = node.args[0]
        with graph.graph.inserting_after(tensor_node):
            new_node = graph.graph.call_function(
                torch.ops.aten.sym_size.int, args=node.args)
            new_node.meta = node.meta.copy()
        node.replace_all_uses_with(new_node)
        graph.graph.erase_node(node)

Test plan

• test_sym_size_in_producer_subgraph — verifies sym_size nodes are in the producer subgraph, not the consumer, and functional correctness is preserved
• test_symint_crosses_split_boundary — verifies SymInt from torch.compile + mark_dynamic crosses split boundaries without errors
• Existing tests (test_getitem_moved_to_producer_subgraph, test_no_tuple_inputs_with_multiple_consumers, test_consecutive_ops_in_split) continue to pass

---

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• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
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• (Optional) Release notes update. If your change is user facing, please update the release notes draft in the Google Doc.

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[gemini-code-assist]

Code Review

The pull request introduces functionality to handle tensor size operations (sym_size) across graph split boundaries in PyTorch's FX graph. This is crucial for preventing issues where torch.Size is not fully supported as a submodule output when sym_size is in one subgraph and its consumer is in another. The changes include adding helper functions _is_sym_size_op and _move_sym_size_nodes_for_split in vllm/compilation/backends.py, and integrating the latter into the split_graph function. New tests in tests/compile/test_graph_partition.py validate this behavior. The overall approach is sound and addresses a known limitation in PyTorch 2.

[mergify]

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[ProExpertProg]

Thanks for this fix!

[ProExpertProg]

Looks overall good to me but will defer to torch.compile folks

[mergify]

Hi @fxdawnn, the pre-commit checks have failed. Please run:

uv pip install pre-commit
pre-commit install
pre-commit run --all-files

Then, commit the changes and push to your branch.

For future commits, pre-commit will run automatically on changed files before each commit.

Tip

Is mypy or markdownlint failing?

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# For mypy (substitute "3.10" with the failing version if needed)
pre-commit run --hook-stage manual mypy-3.10
# For markdownlint
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[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @fxdawnn.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @fxdawnn.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork