Py2 -> py3 for caffe2/caffe2/contrib/tensorboard (#32882)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/32882
Update tensorboard binary and unit tests to python 3
Test Plan:
```
> buck test //caffe2/caffe2/contrib/tensorboard:tensorboard_test
```
```
> buck test //caffe2/caffe2/contrib/tensorboard:tensorboard_exporter_test
```
Reviewed By: sanekmelnikov
Differential Revision: D19670873
fbshipit-source-id: f5eb65ccbb4ecfdc801b9fa05a60d4c5c29dc428
