update the requirements and fix the `_expand_mask` import issue

[maulikmadhavi]

• Update the requirement list (independent from transformer version)
• Fix the issue of _expand_mask #571

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[salesforce-cla[bot]]

Thanks for the contribution! Unfortunately we can't verify the commit author(s): Junnan Li <j***@s***.com>. One possible solution is to add that email to your GitHub account. Alternatively you can change your commits to another email and force push the change. After getting your commits associated with your GitHub account, refresh the status of this Pull Request.

[salesforce-cla[bot]]

Thanks for the contribution! Before we can merge this, we need @dxli94 @schhoi to sign the Salesforce Inc. Contributor License Agreement.