Adding Type Double to Caffe2 Mean Op
Summary: Adding support for type double to caffe2 MeanOp and MeanGradientOp.
Test Plan:
All tests passed.
Example FBL job failed without this diff:
f221169563
Error message:
```
c10::Error: [enforce fail at mean_op.h:72] . Mean operator only supports 32-bit float, but input was of type double (Error from operator:
input: "dpsgd_8/Copy_3" input: "dpsgd_8/Copy_4" output: "dpsgd_8/Mean_2" name: "" type: "Mean" device_option { device_type: 0 device_id: 0 })
```
Example FBL job is running without failure with the canary package built from this diff:
f221468723
Reviewed By: chenshouyuan
Differential Revision: D23956222
fbshipit-source-id: 6c81bbc390d812ae0ac235e7d025141c8402def1
