Delete zero_dim_dispatch_when_scalar

[ezyang]

This is for legacy TH. It is currently used by the following TH functions:

• _th_scatter_ Migrate scatter and scatter_ from the TH to Aten (CUDA) #24621 Migrate scatter and scatter_ from the TH to Aten (CPU) #24757
• _th_fmod Migrate fmod and fmod_ from the TH to Aten (CUDA) #24565 Migrate fmod and fmod_ from the TH to Aten (CPU) #24701
• _th_remainder Migrate remainder and remainder_ from the TH to Aten (CPU) #24753 Migrate remainder and remainder_ from the TH to Aten (CUDA) #24615

cc @ezyang @gchanan @zou3519

[ezyang]

An alternative for deleting this codegen is to just move the codegen into the implementation (which is a shorter thing to do).

[Baranowski]

I will work on moving this codegen into the implementation.

[Baranowski]

@ezyang This is only used by _th_scatter_.

_th_fmod and _th_remainder both have

        - arg: THTensor* self
          broadcast: other inplace fallback
        - THTensor* other

which makes handle_zero_dim() ignore them due to:

        if broadcasts_arg:
            return []

_th_scatter_ is already being migrated to ATen, so I propose that I will abandon this issue.

[ezyang]

ok sgtm. when scatter lands we can pick this up again.