Use external_deps for sentencepiece

as title

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

[ghstack-poisoned]

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(