The [original PR](https://github.com/ray-project/ray/pull/21864) was [reverted](https://github.com/ray-project/ray/pull/22117) because it caused `torch` (more specifically, `torch>=1.8.1`) to be required to use `ray.train`.
```
| File "ray_sgd_training.py", line 18, in <module>
| from ray import train
| File "/home/ray/anaconda3/lib/python3.7/site-packages/ray/train/__init__.py", line 2, in <module>
| from ray.train.callbacks import TrainingCallback
| File "/home/ray/anaconda3/lib/python3.7/site-packages/ray/train/callbacks/__init__.py", line 8, in <module>
| from ray.train.callbacks.profile import TorchTensorboardProfilerCallback
| File "/home/ray/anaconda3/lib/python3.7/site-packages/ray/train/callbacks/profile.py", line 6, in <module>
| from torch.profiler import profile
| ModuleNotFoundError: No module named 'torch.profiler'
```
A [minimal installation test suite](https://github.com/ray-project/ray/pull/22300) was added to detect this. Further, in this PR we make the following changes:
1. Move `TorchWorkerProfiler` to `ray.train.torch` so all torch imports are centralized.
2. Add import validation logic to `TorchWorkerProfiler.__init__` so an exception will only be raised if the user tries to initialize a `TorchWorkerProfiler` without having a valid version of `torch` installed:
```
>>> import ray
>>> import ray.train
>>> import ray.train.torch
>>> from ray.train.torch import TorchWorkerProfiler
>>> twp = TorchWorkerProfiler()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/Users/matt/workspace/ray/python/ray/train/torch.py", line 365, in __init__
"Torch Profiler requires torch>=1.8.1. "
ImportError: Torch Profiler requires torch>=1.8.1. Run `pip install 'torch>=1.8.1'` to use TorchWorkerProfiler.
```
Implement a TorchTensorboardProfilerCallback and corresponding TorchWorkerProfiler to support distributed PyTorch Profiler With TensorBoard integration.
