Maskrcnn training script updates

[ChaiBapchya]

Add fusion flag and log speed at the end of each training epoch

For logging
Previously output was

[Epoch 3] Training cost: 557.830, RPN_Conf=0.044,RPN_SmoothL1=0.039,RCNN_CrossEntropy=0.533,RCNN_SmoothL1=0.273,RCNN_Mask=0.600
[1,0]<stderr>:INFO:root:[Epoch 3] Validation Inference cost: 33.576

Now

[Epoch 3] Training cost: 557.830, Speed: 209.766 samples/sec, RPN_Conf=0.044,RPN_SmoothL1=0.039,RCNN_CrossEntropy=0.533,RCNN_SmoothL1=0.273,RCNN_Mask=0.600
[1,0]<stderr>:INFO:root:[Epoch 3] Validation Inference cost: 33.576

[github-actions]

Job PR-1595-ca59357 is done.
Docs are uploaded to http://gluon-vision-staging.s3-website-us-west-2.amazonaws.com/PR-1595/ca59357/index.html

[github-actions]

Job PR-1595-d4e5193 is done.
Docs are uploaded to http://gluon-vision-staging.s3-website-us-west-2.amazonaws.com/PR-1595/d4e5193/index.html