[Bonsai] Enable Regression for PyTorch

[siddhant-0707]

• Added regression capability for pytorch implementation of Bonsai
• Similar to tf implementation, you can train model specifically for regression using -regression True flag
• Tested on boston-housing and mg regression datasets

Hey @harsha-simhadri, @aayan636 please take a look

[siddhant-0707]

@microsoft-github-policy-service agree