Basic APIs and Architecture Ready

This is an alpha development release.

Release note:

Core APIs ready:

• Graph [backward, forward]
• Tensor
• Operator
• Symbol

Operators Ready (Grad compute not ready of some operators):

• Unary operators
• Binary operators
• Reduce operators
• Activation operators
• Matrix multiplication operator
• 2D convolutional operator
• 2D Pooling operators

Next phase goal:

• Graph memory allocation optimization
• Operator grad compute
• Add operators:
  • Depth-wise convolutional operator
  • Batch normalization operator
• Add optimizer:
  • Adam
• Serrano-tool:
  • Convert Keras model