Add typing for latest versions compatibility

[AlexKay28]

Added explicit typing into "bf16" condition usage section.

I have seen a lot of messages in discord chats and struggled personally with this. This PR doesn't change anything in functionality, but just makes code more clear and friendly to the older versions of torch packages

[BlinkDL]

It works if you use deepspeed==0.7.0 pytorch-lightning==1.9.2 please try :)

[AlexKay28]

@BlinkDL yes, it works, i knew it :) But I noticed you don't provide in the project requirements.txt file or something + it might be not possible to install needed versions in some environments, so I decided to add this typing to avoid such kind of problems.

Feel free to close this issue without applying changes if you think it's redundant, but I have seen some messages from people with this problem :/ I just thought it could make the code safer and that's it)