Add PEP 396 __version__ attribute to torch_tb_profiler module

[joyceerhl]

Background

The Python extension for VS Code needs to ensure that users upgrade to the latest version of the torch_tb_profiler module so that users can take advantage of the jump-to-source support being added in v0.2.0.

We currently check version info for Python modules using the __version__ attribute specified in PEP 396 (code pointer here).

However, the __version__ attribute is currently missing from torch_tb_profiler v0.1.0 and 0.2.0rc2:

Description of changes

This PR makes __init__.py the single source of truth for version info. Specifically, it moves the version number from version.txt to __init__.py, and setup.py then reads the version number out of __init__.py. This is the first approach described in https://packaging.python.org/guides/single-sourcing-package-version/ and also the approach adopted by pip itself:

Test plan

To verify that this change works and doesn't break setup, I did the following:

1. Made changes to the source
2. python3 -m pip install /workspaces/kineto/tb_plugin to install the package locally
3. python3 -c "import torch_tb_profiler; print(torch_tb_profiler.__version__)" now prints out the version number:

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