Add tests for flash_attention_backward perf model (PR #512)

[Copilot]

PR #512 overhauled flash_attention_backward — reparenting it from flash_attention to SDPA, adding a proper get_param_details, and registering flash_attn::_flash_attn_backward in the op mapping — but shipped without tests.

New test file: tests/test_flash_attention_backward.py

• Mapping & categorization — flash_attn::_flash_attn_backward resolves to flash_attention_backward and categorizes as SDPA_bwd
• Class hierarchy — inherits SDPA, not flash_attention
• get_param_details argument indexing — dout at index 0; q/k/v correctly read from indices 1/2/3
• get_param_details concrete inputs — dropout from concrete[8], causal from concrete[10]; both default safely when Concrete Inputs is absent
• __init__ attribute — self.d_h is set to self.d_h_qk (required by get_simulation_time_bwd_func)
• flops() — delegates to flops_bwd(); backward > forward for identical shapes; non-causal > causal
• bytes() — zero-arg form resolves bytes_per_element from param_details dtype; explicit bpe delegates to bytes_bwd()
• fa flag — type(self).__name__ check in get_simulation_time_bwd correctly identifies flash_attention_backward as a flash-attention op

[spandoesai]

Should probably wait until #582 closes since that PR makes changes to the reference reports? @unagiboi could you merge that one first?

[unagiboi]

@spandoesai #582 has been merged.

[spandoesai]

Thanks, @gabeweisz could you rebase and try the tests again?

[gabeweisz]

Merged main. This particular PR does not use reference reports, it just adds additional tests. All tests pass after merging main