torchbench is now a library (#1933)
Summary:
```python
import torchbenchmark.models.densenet121
model, example_inputs = torchbenchmark.models.densenet121.Model(test="eval", device="cuda", batch_size=1).get_module()
model(*example_inputs)
```
So making the above example work with `import torchbenchmark` alone is a bit trickier there's a lot of relative imports that need to be fixed in pretty much every single file
I also added a simple `test_imports.py` file to make sure this doesn't break anything, I can setup a standalone github action or plug into an existing one (would rather do the latter to avoid another job that needs to run `install.py` again
This also solves voznesenskym favorite design pattern from https://github.com/pytorch/pytorch/blob/main/benchmarks/dynamo/torchbench.py
```python
for torchbench_dir in (
"./torchbenchmark",
"../torchbenchmark",
"../torchbench",
"../benchmark",
"../../torchbenchmark",
"../../torchbench",
"../../benchmark",
):
```
Pull Request resolved: https://github.com/pytorch/benchmark/pull/1933
Reviewed By: xuzhao9
Differential Revision: D49608012
Pulled By: msaroufim
fbshipit-source-id: 382a31f2c5a464608faa730ae15148f59a3e7484
