[examples] better PL version check
`pkg_resources.require(f"{pkg}>={min_ver}")` does a great job of checking the minimal required versions at runtime, but I wasn't aware that it checks that the dependencies meet their requirements too! So I started getting a false alarm about needing `pytorch-lightning=1.0.4` when I already had a higher version. 

The problem was in:
```
$ python -c 'import pkg_resources; pkg_resources.require("pytorch_lightning>=1.0.4")'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/home/stas/anaconda3/envs/main-38/lib/python3.8/site-packages/pkg_resources/__init__.py", line 884, in require
    needed = self.resolve(parse_requirements(requirements))
  File "/home/stas/anaconda3/envs/main-38/lib/python3.8/site-packages/pkg_resources/__init__.py", line 775, in resolve
    raise VersionConflict(dist, req).with_context(dependent_req)
pkg_resources.ContextualVersionConflict: (torch 1.8.0.dev20201106+cu110 (/mnt/nvme1/anaconda3/envs/main-38/lib/python3.8/site-packages), Requirement.parse('torch<1.8,>=1.3'), {'pytorch-lightning'})
```

Long story short, currently PL explicitly excludes pytorch-1.8 in its dependency list: https://github.com/PyTorchLightning/pytorch-lightning/issues/4596 - which leads to this problem. When I upgrade PL pip uninstalls `pytorch-1.8` - thanks, but no thanks - rtx-3090 doesn't work with pytorch < 1.8. So I install it back and now I get the failure above. Except in the current code it's masked by the `try/except` block which hides the actual problem. So this is not good.

This PR rewrites the check in a way that doesn't check whether the dependencies of the package in questions are in order, and only checks that the minimal version is correct.

@sshleifer  