Revert "[Kernel] Use flash-attn for decoding (#3648)" (#4820)

Lora 3 & 4 test seems to have illegal memory access failure after this commit;

[2024-05-14 23:51:18,182 E 22 22] logging.cc:101: Unhandled exception: N3c105ErrorE. what(): CUDA error: an illegal memory access was encountered
<br class="Apple-interchange-newline">
Exmaple: https://buildkite.com/vllm/ci/builds/7382#018f793d-1527-4e1c-ab59-c3a34ec55241

This reverts commit 1356df5.

FILL IN THE PR DESCRIPTION HERE

FIX #xxxx (link existing issues this PR will resolve)

tests/models/test_big_models.py

@@ -12,7 +12,7 @@ MODELS = [
     # "Deci/DeciLM-7b",  # Broken
     # "tiiuae/falcon-7b",  # Broken
     "EleutherAI/gpt-j-6b",
-    # "mosaicml/mpt-7b",  # Broken
+    "mosaicml/mpt-7b",
     # "Qwen/Qwen1.5-0.5B"  # Broken,
 ]