Add direct copy fast path for portable copy op

Summary:
The PR adds a direct memcpy fast-path for portable copy and copy_ ops. This speeds up copy significantly in cases where no broadcasting is needed.

This is most noticable when copying buffer mutations back, such as transformer KV cache when managing the cache as a mutable buffer. Prior to this change, an encoder/decoder model was taking ~25% of the total runtime copying KV cache back after permuting. With this change, the copy becomes significantly cheaper.

I benchmarked a simple model on S23 and Pixel 5:
```
class TestModel(torch.nn.Module):
    def __init__(self):
        super().__init__()
        self.register_buffer("buffer", torch.zeros((2, 10, 1024, 1024)))

    def forward(self, x):
        self.buffer.add_(x)
        return self.buffer


model = TestModel()
inputs = (torch.randn(2, 10, 1024, 1024),)

lowered = to_edge_transform_and_lower(
    torch.export.export(model, inputs),
    partitioner=[XnnpackPartitioner()],
).to_executorch()
```

S23, average of 50 runs, time in copy_:
4.1ms vs 22.3ms

Pixel 5, average of 50 runs, time in copy_:
12.1ms vs 66.6ms

This is approximately a ~5.5x speedup of the copy operator.

Differential Revision: D73656456

Author: GregoryComer

kernels/portable/cpu/op_copy.cpp

@@ -46,17 +46,22 @@ Tensor& copy_out(
   // @lint-ignore CLANGTIDY facebook-hte-CArray
   static constexpr const char op_name[] = "copy.out";
 
-  ET_SWITCH_REALHBBF16_TYPES(in.scalar_type(), ctx, "copy.out", CTYPE, [&]() {
-    utils::apply_bitensor_elementwise_fn<CTYPE, op_name>(
-        [](ET_UNUSED const CTYPE _, const CTYPE val_src) { return val_src; },
-        ctx,
-        in,
-        utils::SupportedTensorDtypes::REALHBBF16,
-        src,
-        utils::SupportedTensorDtypes::REALHBBF16,
-        out,
-        utils::SupportedTensorDtypes::REALHBBF16);
-  });
+  // Use direct copy fast path if broadcast is not needed and tensors are non-empty
+  if (tensors_have_same_shape(out, src) && src.numel() > 0) {
+      std::memcpy(out.mutable_data_ptr(), src.const_data_ptr(), src.nbytes());
+  } else {
+      ET_SWITCH_REALHBBF16_TYPES(in.scalar_type(), ctx, "copy.out", CTYPE, [&]() {
+        utils::apply_bitensor_elementwise_fn<CTYPE, op_name>(
+            [](ET_UNUSED const CTYPE _, const CTYPE val_src) { return val_src; },
+            ctx,
+            in,
+            utils::SupportedTensorDtypes::REALHBBF16,
+            src,
+            utils::SupportedTensorDtypes::REALHBBF16,
+            out,
+            utils::SupportedTensorDtypes::REALHBBF16);
+      });
+    }
 
   return out;
 }
@@ -79,17 +84,22 @@ Tensor& copy_(
   // @lint-ignore CLANGTIDY facebook-hte-CArray
   static constexpr const char op_name[] = "copy_";
 
-  ET_SWITCH_REALHBBF16_TYPES(in.scalar_type(), ctx, "copy_", CTYPE, [&]() {
-    utils::apply_bitensor_elementwise_fn<CTYPE, op_name>(
-        [](ET_UNUSED const CTYPE _, const CTYPE val_src) { return val_src; },
-        ctx,
-        in,
-        utils::SupportedTensorDtypes::REALHBBF16,
-        src,
-        utils::SupportedTensorDtypes::REALHBBF16,
-        in,
-        utils::SupportedTensorDtypes::REALHBBF16);
-  });
+  // Use direct copy fast path if broadcast is not needed and tensors are non-empty
+  if (tensors_have_same_shape(in, src) && src.numel() > 0) {
+      std::memcpy(in.mutable_data_ptr(), src.const_data_ptr(), in.nbytes());
+  } else {
+      ET_SWITCH_REALHBBF16_TYPES(in.scalar_type(), ctx, "copy_", CTYPE, [&]() {
+        utils::apply_bitensor_elementwise_fn<CTYPE, op_name>(
+            [](ET_UNUSED const CTYPE _, const CTYPE val_src) { return val_src; },
+            ctx,
+            in,
+            utils::SupportedTensorDtypes::REALHBBF16,
+            src,
+            utils::SupportedTensorDtypes::REALHBBF16,
+            in,
+            utils::SupportedTensorDtypes::REALHBBF16);
+      });
+  }
 
   return in;
 }