add pytorch backend flag

[gusye1234]

see #62

[abrown]

@gusye1234, thanks for the contribution, but I don't think this is the right way to add this support:

• first, we need to enable the flag in the WIT/WITX specification, e.g., here
• then, we need to re-generate this bindings file so that it includes the new variant (note how the header of this file marks it as auto-generated)

As a separate issue, the WASI ecosystem has begun a transition from WITX specification files to WIT specification files, which means that the generation step for this file should switch to using WIT-based tools (i.e., wit-bindgen). I don't really expect you to have to worry too much about that (@brianjjones, do you want to take a look?) so I will create a separate issue for that: #64. @gusye1234, does that make sense? I am on Zulip if you want to discuss in more detail.

[abrown]

(And apologies for contradicting myself in #62... I think I assumed this option was fine to add but now, looking at it more clearly, I think we need to be able to re-generated that file).

[gusye1234]

Cool! Your suggestion to sync between this auto-generated file and WIT specification file is better.
I should pull a request here and close this one.