Issue resolved now,
The problem was with safari browser. Whenever I am opening your given URL “https://index.taskcluster.net/v1/task/project.deepspeech.tensorflow.pip.master.osx/artifacts/public/convert_graphdef_memmapped_format” in Safari it is downloading convert_graphdef_memmapped_format file in .dms format. Then I tried the same url into chrome browser and it is downloading convert_graphdef_memmapped_format file in the same format what I was expecting.
(deepspeech_env) Rahuls-MacBook-Pro-2:DeepSpeech-master rahulsharma$ ./convert_graphdef_memmapped_format --in_graph=/Users/rahulsharma/Documents/Tuple_Work/Speech/DeepSpeech-master/output_graph.pb --out_graph=/Users/rahulsharma/Documents/Tuple_Work/Speech/DeepSpeech-master/pbmm_file_generation/output_graph.pbmm
2019-06-14 22:23:45.413515: I tensorflow/contrib/util/convert_graphdef_memmapped_format_lib.cc:171] Converted 23 nodes
@lissyx Thank you so much for the help