[ROCm] Disable chunked prefill/prefix caching when running MLA on non-cuda platforms

[SageMoore]

flash_attn_varlen_func in upstream flash-attn does not support the return_softmax_lse argument. This PR works around that issue by explicitly disabling chunked prefill and prefix caching on non-cuda platforms.

Here are the results from running llmeval with deepseek-ai/DeepSeek-Coder-V2-Lite-Instruct on an AMD system:

|Tasks|Version|     Filter     |n-shot|  Metric   |   |Value |   |Stderr|
|-----|------:|----------------|-----:|-----------|---|-----:|---|-----:|
|gsm8k|      3|flexible-extract|     5|exact_match|↑  |0.7657|±  |0.0117|
|     |       |strict-match    |     5|exact_match|↑  |0.7453|±  |0.0120|

Here are the results from the same model on an H100 system without this PR

|Tasks|Version|     Filter     |n-shot|  Metric   |   |Value |   |Stderr|
|-----|------:|----------------|-----:|-----------|---|-----:|---|-----:|
|gsm8k|      3|flexible-extract|     5|exact_match|↑  |0.7642|±  |0.0117|
|     |       |strict-match    |     5|exact_match|↑  |0.7468|±  |0.0120|

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[mgoin]

Nice work, LGTM