Add save_checkpoint arg for TIMM training to simplify validation

[ZhengHongming888]

What does this PR do?

In order to simplify the validation testing effort here one arg is added to choose whether save_checkpoint is executed or not to save the validation time. The command below in default is not saving checkpoint per epoch.

python train_hpu_graph.py
--data-dir ./
--dataset hfds/johnowhitaker/imagenette2-320
--device 'hpu'
--model resnet50.a1_in1k
--train-split train
--val-split train
--dataset-download

if you want to save checkpoint per epoch you need add one more argument " --save_checkpoint " in the command.

In addition the README.md also is simplified for validation target.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests?

[jiminha]

LGTM!