Revert "[Inductor] FlexAttention backward kernel optimization (pytorch#127208)"

This reverts commit f717131.

Reverted pytorch#127208 on behalf of https://github.com/yanboliang due to test_flex_attention is failing internally ([comment](pytorch#127208 (comment)))

Author: pytorchmergebot

test/inductor/test_flex_attention.py

@@ -144,8 +144,6 @@ def _check_equal(
     ):
         compiled_error = (golden_out - compiled_out).abs().mean()
         ref_error = (golden_out - ref_out).abs().mean()
-        if torch.isnan(compiled_error).any() and not torch.isnan(ref_error).any():
-            self.assertTrue(False, "Output/Grad with NaN")
         if compiled_error > ref_error * fudge_factor:
             name = tensor_name if tensor_name is not None else ""
             msg = f"{name} Compiled error {compiled_error} is greater than ref error {ref_error} by more than {fudge_factor}X."
@@ -197,7 +195,7 @@ def run_test(
             self._check_equal(
                 k_gold.grad, k_ref.grad, k.grad, k_fudge_factor, "Grad_Key"
             )
-            v_fudge_factor = 4 * fudge_factor
+            v_fudge_factor = 8 * fudge_factor
             self._check_equal(
                 v_gold.grad, v_ref.grad, v.grad, v_fudge_factor, "Grad_Value"
             )