Narrow the aten_device torch dependency to the ATen core (pytorch#22146)

`aten_device.h` uses `c10::Device` and the c10 diagnostic macros, so the
ATen and c10 core headers are all it needs. The target asked for the
whole `libtorch` external dep instead.

That matters because `:aten_bridge` exports `:aten_device`, and
`:aten_bridge` is a portable-mode target with public visibility. Any
application linking it therefore picked up the full `libtorch`. In build
environments where the `libtorch` external dep resolves to a library
that also registers every ATen operator, that collides with the
application's own operator registration library and the link fails with
duplicate symbols such as `at::compositeimplicitautograd::_cast_Byte`.
On Android the same thing shows up one level earlier, as two rules
producing the same shared library name.

`:aten_bridge` already declares `torch-core-cpp` for exactly this
reason, and every other `exported_external_deps = ["libtorch"]` in this
repo is guarded by `if aten_mode`. This change makes `:aten_device`
match.

The dep stays exported because `:aten_device` is header only and its
consumers, including `aten_bridge.h`, include the header directly.

## Test plan

In the open-source build both `libtorch` and `torch-core-cpp` map to the
same target, so this is a no-op here and CI should stay green.

The difference was checked in a build environment where the two names
resolve to different targets: a dependency query returns a path from the
`libtorch` target to the full operator registration library, and returns
nothing from the `torch-core-cpp` target to the same library.

Author: shoumikhin

extension/aten_util/targets.bzl

@@ -17,8 +17,12 @@ def define_common_targets():
             "//executorch/runtime/core/portable_type:device",
             "//executorch/runtime/platform:platform",
         ],
+        # aten_device.h needs c10::Device and nothing else. The full libtorch
+        # also registers every ATen operator, which duplicates the
+        # selective-build operator library in apps that reach this target
+        # through :aten_bridge, and :aten_bridge is portable mode.
         exported_external_deps = [
-            "libtorch",
+            "torch-core-cpp",
         ],
     )