Fix LocalBuilder on macos with python 3.8. (apache#6083)

Python 3.8 changes the default way multiprocessing creates new processes
on macOS from forking to spawing. Spawning requires all objects to be
picklable. Nested functions and lambdas are not picklable, so this
commit fixes the one instance of nested functions in the codebase that
was causing issues.

python/tvm/autotvm/measure/measure_methods.py

@@ -89,7 +89,7 @@ def __init__(self, timeout=10, n_parallel=None, build_func='default'):
                 build_func = ndk.create_shared
             else:
                 raise ValueError("Invalid build_func" + build_func)
-        self.build_func = _wrap_build_func(build_func)
+        self.build_func = _WrappedBuildFunc(build_func)
         self.executor = LocalExecutor(timeout=timeout)
         self.tmp_dir = tempfile.mkdtemp()
 
@@ -390,25 +390,29 @@ def _build_func_common(measure_input, check_gpu=None, cuda_arch=None, build_opti
     return func, tuple((get_const_tuple(x.shape), x.dtype) for x in args)
 
 
-def _wrap_build_func(build_func):
+class _WrappedBuildFunc():
     """
     Wrap build_func to a function that can be used in measure.
 
+    Note: this is a class instead of a closure so that it can be pickled when
+    using multiprocessing.
+
     Parameters
     ----------
     build_func : The compilation function
         We expect fcompile to contain an attr "output_format"
 
     Returns
     -------
-    wrapped_build_func : function
+    wrapped_build_func : callable
         The wrapped build function
     """
-    if not hasattr(build_func, "output_format"):
-        raise AttributeError("Expect build_func to have the attribute output_format.")
-    output_format = build_func.output_format
+    def __init__(self, build_func):
+        if not hasattr(build_func, "output_format"):
+            raise AttributeError("Expect build_func to have the attribute output_format.")
+        self.build_func = build_func
 
-    def _wrapped(measure_input, tmp_dir, **kwargs):
+    def __call__(self, measure_input, tmp_dir, **kwargs):
         """
         Wrapped build func.
 
@@ -423,15 +427,13 @@ def _wrapped(measure_input, tmp_dir, **kwargs):
         tic = time.time()
         try:
             filename = os.path.join(tmp_dir, "tmp_func_%0x.%s" % (
-                getrandbits(64), output_format))
+                getrandbits(64), self.build_func.output_format))
             # TODO(tvm-team) consider linline _build_func_common
             func, arg_info = _build_func_common(measure_input, **kwargs)
-            func.export_library(filename, build_func)
+            func.export_library(filename, self.build_func)
         except Exception as e:  # pylint: disable=broad-except
             return BuildResult(None, None, e, time.time() - tic)
         return BuildResult(filename, arg_info, None, time.time() - tic)
-    return _wrapped
-
 
 def run_through_rpc(measure_input, build_result,
                     number, repeat, min_repeat_ms, cooldown_interval,