Use external_deps for sentencepiece

Pull Request resolved: #4269

as title
ghstack-source-id: 234090425

Differential Revision: [D59770172](https://our.internmc.facebook.com/intern/diff/D59770172/)

Author: helunwencser

extension/llm/tokenizer/targets.bzl

@@ -20,8 +20,9 @@ def define_common_targets():
             "//bento_kernels/...",
         ],
         _is_external_target = True,
-        # TODO: Define an external_deps entry for sentencepiece instead of pointing to an fbsource path.
-        deps = [] if runtime.is_oss else ["fbsource//third-party/pypi/sentencepiece:sentencepiece"],
+        external_deps = [
+            "sentencepiece-py",
+        ],
     )
 
     runtime.python_binary(

shim/xplat/executorch/build/env_interface.bzl

@@ -42,6 +42,7 @@ _EXTERNAL_DEPS = {
     "prettytable": "//third-party:prettytable",
     "pybind11": "//third-party:pybind11",
     "re2": [],  # TODO(larryliu0820): Add support
+    "sentencepiece-py": [],
     # Core C++ PyTorch functionality like Tensor and ScalarType.
     "torch-core-cpp": "//third-party:libtorch",
     "torchgen": "//third-party:torchgen",