examples/run_summarization_no_trainer: fixed incorrect param to hasattr
Author: rahular
Pull request: yes

# What does this PR do?

Fixes a small bug in `examples/run_summarization_no_trainer.py` which resulted in the script not checkpointing models even if the correct argument was passed from CLI.

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@sgugger @patil-suraj 

Comments:
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_18720). All of your documentation changes will be reflected on that endpoint.

Would you like to take a look at this, @muellerzr?

@rahular this isn't quite the right solution. What this needs to check here is whether or not the checkpointing steps passed were either `"epoch"` or a digit. So what this currently does is ignore if epoch was actually passed. With this knowledge in mind do you want to give it another go? I'll be able to give a more detailed review if you're stuck etc in a few hours 😄 

Hi @muellerzr, `checkpointing_steps = args.checkpointing_steps` should already take care of the `epoch` case, if I'm not mistaken. Here's the flow in comments.
```
    if hasattr(args, "checkpointing_steps"):                        # check if `args` has `checkpointing_steps`
        checkpointing_steps = args.checkpointing_steps              # stores whatever was passed as a local variable (including `epoch`)  
        if args.checkpointing_steps.isdigit():                      # check if the passed argument is a digit
            checkpointing_steps = int(args.checkpointing_steps)     # overwrite the local variable with the digit after casting
    else:                                                           # local variable is `None` if `args` does not have `checkpointing_steps`
        checkpointing_steps = None
```
Let me know what you think.

@rahular good point! I think in that case something like the following chain may be how we want to do this:

```python
checkpointing_steps = args.checkpointing_steps
if checkpointing_steps is not None and checkpointing_steps.isdigit():
        checkpointing_steps = int(checkpointing_steps)
```
Since it's always present (it's part of the arg parser) and we really care if it's not None or if it's a digit. Does this make sense to you as well?

(And the default is `None` anyways)

@muellerzr makes sense! Have simplified it and pushed.

Also make sure to run `make style; make quality` from the base directory of your fork, to fix the style failure

@muellerzr Sure I can take a look at other trainers as well!

@muellerzr let's close this PR and I will update the other trainers and create a new request.