Fix linear regression tutorial (#6978)
Fix linear regression tutorial (#6978)

* Fix linear regression tutorial

- Print training logs in notebook.
- Switch from accuracy to mse for validation metrics. Accuracy doesn't make much sense for regression.
- Lower the number of epochs to 50. Number of epochs in unnecessarily huge (1000) consuming too much time. Training converges in around 30 epochs.

* Fix indentation.

Guneet-Dhillon/mxnet,Guneet-Dhillon/mxnet,Guneet-Dhillon/mxnet,Guneet-Dhillon/mxnet,Guneet-Dhillon/mxnet,Guneet-Dhillon/mxnet,Guneet-Dhillon/mxnet,Guneet-Dhillon/mxnet,Guneet-Dhillon/mxnet