Pass using_checkpoint flag from ViTUnicomExtractor to VisionTransformer

[korotaS]

• I've checked contribution guide.

This PR fixes an error which was first discovered in #592 about exporting ViTUnicomExtractor to ONNX. The fix is simple but there is a question - is it enough to pass only using_checkpoint param or we can pass arbitrary params as **kwargs into VisionTransformer class? Also, do we need to test vitb16_unicom, vitl14_unicom and vitl14_336px_unicom or is it enough to test only one vitb32_unicom?

[AlekseySh]

is it enough to pass only using_checkpoint param or we can pass arbitrary params as **kwargs into VisionTransformer class?

I think it's okay to keep only using_checkpoint at the moment

Also, do we need to test vitb16_unicom, vitl14_unicom and vitl14_336px_unicom or is it enough to test only one vitb32_unicom?

I think we don't want to have too much tests. If you tested them locally it's enough

[AlekseySh]

• Reminder: update the issue with an instruction after we merge it

[AlekseySh]

LGTM, thx for the contribution!