Added GQA support for CUDNNAttention backend

[MaciejBalaNV]

This PR adds GQA support for CUDNNAttentionBackend, similar to how SDPA was handled here: #3728

Purpose

Enabling models to use CUDNNAttentionBackend with GQA. One example is Bagel, other is Cosmos3 that is being added here: #3454

Test Plan

Since CUDNNAttentionBackend or GQA for SDPA didn't contain any tests, not extending it here, since it's out of scope of the PR and the change is very simple.

Test Result

No relevant test results

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• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan. Please provide the test scripts & test commands. Please state the reasons if your codes don't require additional test scripts. For test file guidelines, please check the test style doc
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[MaciejBalaNV]

CC @alex-jw-brooks