Bundle Specification Needs Required Packages

[ericspod]

Is your feature request related to a problem? Please describe.
Bundles can be specified to depend on optional packages through the optional_packages_version in metadata.json. Some bundles may require certain packages so a required_packages_version may be needed, or a change in name for optional_packages_version.

Describe the solution you'd like
Some way of specifying what packages other Pytorch, Numpy, and MONAI are needed by a bundle.

Describe alternatives you've considered
I've used optional_packages_version to specify needed packages but this doesn't seem adequate.

[ericspod]

I don't think this is a really high priority item, but we should update the metadata schema some time anyway.

[KumoLiu]

Hi @ericspod, for now, if optional_packages_version is in "metadata.json", it will add these packages to requirements.txt. Doesn't this currently meet the request, or do you think it would be better to change the name to required_packages_version?
https://github.com/Project-MONAI/model-zoo/blob/8845f3a04766e254b156c23fc4d2f91659c056d4/ci/get_bundle_requirements.py#L39

[ericspod]

The issue is that there are required packages that a bundle absolutely needs and packages that would nice to have but not strictly needed. These two aren't differentiated in the json file and I felt there should be, or we change the name to required_packages_version to make it clear these packages are needed and not worry about having things optional.