Adding back a check for empty library (apache#49)

* Adding back a check for empty library

* Adding test for empty Module compilation

python/tvm/module.py

@@ -116,6 +116,19 @@ def export_library(self,
         if isinstance(file_name, Path):
             file_name = str(file_name)
 
+        if self.is_empty():
+            logging.info("The lib generated by the NNVM compiler does not contain optimized "
+                         "functions for any operators. This usually happens when an external "
+                         "accelerator, e.g. TensorRT, is employed along with TVM to compile "
+                         "the model, and all the operators in the model are supported by the "
+                         "external accelerator at runtime. Therefore, "
+                         "the NNVM compiler skipped optimizing them at the compile time.")
+            if os.path.isfile(file_name):
+                logging.warning("Lib file %s exists, and will be overwritten by the newly created"
+                                " lib with the same name.", file_name)
+            open(file_name, 'w').close()
+            return
+
         if self.type_key == "stackvm":
             if not file_name.endswith(".stackvm"):
                 raise ValueError("Module[%s]: can only be saved as stackvm format."

tests/python/unittest/test_tensorrt.py

@@ -0,0 +1,35 @@
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The ASF licenses this file
+# to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance
+# with the License.  You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing,
+# software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied.  See the License for the
+# specific language governing permissions and limitations
+# under the License.
+import os
+from unittest import mock
+import tempfile
+
+import tvm
+
+def test_empty_library_export():
+    with tempfile.TemporaryDirectory() as temp_dir:
+        temp_file_path = os.path.join(temp_dir, "tmp_lib")
+        print(temp_file_path)
+        with mock.patch.object(tvm.module.Module, "is_empty") as is_empty_mock:
+            is_empty_mock.return_value = True
+            module = tvm.module.Module(None)
+            module.export_library(temp_file_path)
+        assert(os.path.isfile(temp_file_path))
+
+
+if __name__ == "__main__":
+    test_empty_library_export()