no flag for checkpoint_max_to_keep in model_main_tf2.py

Prerequisites

Please answer the following question for yourself before submitting an issue.

• I checked to make sure that this feature has not been requested already.

1. The entire URL of the file you are using

https://github.com/tensorflow/models/blob/0d13df63c3dee2d226f715fcac39e38008c66b5a/research/object_detection/model_main_tf2.py

2. Describe the feature you request

model_lib_v2.train_loop supports passing in checkpoint_max_to_keep - but the cli script model_main_tf2.py does not have a flag which allows the user to pass this param through.

https://github.com/tensorflow/models/blob/0d13df63c3dee2d226f715fcac39e38008c66b5a/research/object_detection/model_main_tf2.py

https://github.com/tensorflow/models/blob/0d13df63c3dee2d226f715fcac39e38008c66b5a/research/object_detection/model_lib_v2.py#L452

3. Additional context

Exposing this flag will allow users to specify the number of checkpoints they would like to keep while training.

4. Are you willing to contribute it? (Yes or No)

Yes