move distribution.py into distribution package for better scalability

[cxxly]

PR types

Others

PR changes

Others

Describe

We plan to develop probability distribution package, which will add 25 New APIs and corresponding operators. Currently, paddle organizes all distributions in one file python/paddle/distribution.py. It's difficultly to extend this file for adding New API. So we make new a distribution package python/paddle/distribution, move distribution.py into it, and split it into more simple files.

This modification don't change the path of current api，so users and developer are not affected.
For example, the follow sample code can normally run before and after this modification.

import paddle
from paddle.distribution import Normal

# Define a single scalar Normal distribution.
dist = Normal(loc=0., scale=3.)
# Define a batch of two scalar valued Normals.
# The first has mean 1 and standard deviation 11, the second 2 and 22.
dist = Normal(loc=[1., 2.], scale=[11., 22.])
# Get 3 samples, returning a 3 x 2 tensor.
print(dist.sample([3]))

[paddle-bot-old]

Thanks for your contribution!
Please wait for the result of CI firstly. See Paddle CI Manual for details.

[iclementine]

LGTM

[jeff41404]

lgtm

[paddle-bot-old]

Sorry to inform you that a6efaf2's CIs have passed for more than 7 days. To prevent PR conflicts, you need to re-run all CIs manually.

[XiaoguangHu01]

LGTM