Fix operator<< for Half (#2294)

Summary:
Pull Request resolved: #2294

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Reviewed By: mergennachin

Differential Revision: D54638827

fbshipit-source-id: 98e2c0a3bc2804d677cc3d38561e55d6b54d8f7e

Author: manuelcandales

runtime/core/portable_type/half.cpp

@@ -674,7 +674,12 @@ inline Half operator/(int64_t a, Half b) {
 /// NOTE: we do not define comparisons directly and instead rely on the implicit
 /// conversion Half to float.
 
-std::ostream& operator<<(std::ostream& out, const Half& value);
+static inline std::ostream& operator<<(
+    std::ostream& out,
+    const torch::executor::Half& value) {
+  out << (float)value;
+  return out;
+}
 
 } // namespace executor
 } // namespace torch

runtime/core/portable_type/half.h

@@ -40,7 +40,6 @@ def define_common_targets():
     # Set up a specific exported library for scalar_type to avoid circle dependency in ScalarTypeUtil.h
     runtime.cxx_library(
         name = "scalar_type",
-        srcs = ["half.cpp"],
         exported_headers = [
             "bfloat16.h",
             "complex.h",