Add version api

[Yancey1989]

Fixed #2924
We can use the version API as:

>>> print paddle.version.version
0.10.0
>>> print paddle.version.git_commit
e1ac448c3320d8a11051fad63cc17f3803862236

[wangkuiyi]

I have a serious concern about our current versioning approach, which takes all Git commits between 0.9.0 and 0.10.0 as 0.9.0. This is not realistic.

Consider that when errors occur with an experiment, users should report and upload the log so could we help. The printed log should contain the real version/commit id, so could we reproduce the error and locate the bug.

The correct logic should be

if paddle.version.tagged:
   print paddle.version         # 0.10.0, 0.10.0rc1 or something
   print paddle.version.major   # 0
   print paddle.version.minor   # 10
   print paddle.version.release # 0
   print paddle.version.rc      # 1, or 0 if not a release candidate
else:
   print paddle.version        # 1232131abdd3  
   print paddle.version.commit # 1232131abdd3

What do you think?

[Yancey1989]

Thanks for @wangkuiyi , to distinguish taged or nor is a good idea, I only have some small doubt:

1. paddle.version is a Python module, so we can use paddle.version.full_version?
2. Following Semantic Versioning 2.0.0, the version number is made up by MAJOR.MINOR.PATCH
3. If not a taged version, maybe we do not need to print the repeated information, only print the commit id?

So how about the following logic?

if paddle.version.tagged:
   print paddle.version.full_version         # 0.10.0, 0.10.0rc1 or something
   print paddle.version.major   # 0
   print paddle.version.minor   # 10
   print paddle.version.patch # 0
   print paddle.version.rc      # 1, or 0 if not a release candidate
else:
   print paddle.version.commit # 1232131abdd3

[wangkuiyi]

@Yancey1989 Completely agree! Thanks!

[Yancey1989]

Done with @wangkuiyi 's comments.

[typhoonzero]

@Yancey1989 Can you please update this PR and merge the latest changes, so we can merge this.

[Yancey1989]

With markdownlint MD025, there is only one top-level header.

[typhoonzero]

Seem CI failed at:

File "setup.py", line 46, in write_version_py
with open(filename, 'w') as f:
IOError: [Errno 2] No such file or directory: '$/paddle/python/paddle/version.py'

[Yancey1989]

@typhoonzero , I'm debugging with this failed and will update this PR later.

[typhoonzero]

LGTM! This is a awesome feature.