Revert "export.py: fix custom SDPA type conversion logic & re-enable for bflo…" (#1197)

This reverts commit 3aba730.

Author: swolchok

torchchat/export.py

@@ -199,7 +199,7 @@ def forward(self, x, freqs_cis, mask, input_pos=None, cache_lane: int = 0):
                 input_pos[-1].item(),
                 seqlen,
             )
-            output = output.view(bsz, seqlen, self.dim).to(dtype=x.dtype)
+            output = output.view(bsz, seqlen, self.dim).to(dtype=q.dtype)
             return self.wo(output)
 
     def replace_attention_with_custom_sdpa_attention(module: nn.Module):
@@ -291,7 +291,11 @@ def export_for_et(model, device, output_path) -> str:
             model = model.to(dtype=target_precision)
             state_dict_dtype = target_precision
 
-        replace_attention_with_custom_sdpa_attention(model)
+        # Custom SDPA does not work with bfloat16 on CPU currently. (The op doesn't
+        # support anything but bfloat32, and our attempt to use it anyway by converting
+        # to and from float causes other errors.)
+        if target_precision != torch.bfloat16:
+            replace_attention_with_custom_sdpa_attention(model)
 
         with torch.nn.attention.sdpa_kernel(
             [torch.nn.attention.SDPBackend.MATH]