Created by: pangyoki

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APIs

Describe

Implement Probability Distribution related classes.

• Release Distribution abstract base class and its sample, log_prob, entropy, probs methods.
• Move Normal class and Uniform class in paddle/fluid/layers/distributions.py path to paddle/distribution.py.
• Add parameter name of each class API. name is initialized in __init__ function, and every function of the class has its own name.
• Add probs method for Normal class and Uniform class.