remove image top level import by default

[shcheklein]

Followup #23 . I think it breaks tests in Studio since import datachain now raises exception if torch is not installed.

Need more research if this is a proper fix. This is more for awareness.

[mattseddon]

I will separate image and text as per the OP:

from datachain.image import convert_images, ImageFile
from datachain.text import convert_text, 

[shcheklein]

Thanks @mattseddon !