[Bug] [ROCm] Fix AiterFAMetadata Import Error when AITER is not installed

[tjtanaa]

Purpose

Simple fix to the issue raised in #22795

Test Plan

Test Result

(Optional) Documentation Update

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• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
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• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.

[gemini-code-assist]

Code Review

The pull request aims to fix an ImportError for the ROCm AITer FlashAttention backend when the aiter package is not installed. The change correctly adds a check for the aiter package before attempting to import it. However, this fix is incomplete and can lead to a runtime error if the backend is selected for use without aiter being installed. My review includes a comment explaining the issue and suggesting a more robust fix by making all aiter-dependent class definitions conditional, which is a common pattern in this codebase for handling optional dependencies.

[gemini-code-assist]

While this change correctly prevents an ImportError at module load time if aiter is not installed, it can lead to a RuntimeError later.

The classes AiterFlashAttentionMetadata, AiterFlashAttentionMetadataBuilder, AiterFlashAttentionBackend, and AiterFlashAttentionImpl are defined later in this file (from line 207 onwards) and they depend on the aiter package and the custom ops registered within this if block.

If aiter is not installed, this if block is skipped, but the classes are still defined. When AiterFlashAttentionBackend is used, it will try to call ops that were not registered, leading to a crash.

To fix this properly, all the definitions that depend on aiter should be moved inside this if block. This includes lines 204 through 547. This will ensure that the AiterFlashAttentionBackend is only available when aiter is installed, preventing runtime errors.

A similar pattern is used for other attention backends with optional dependencies, like flash_attn.

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