[pytorch] Disable new autograd fallback for mobile builds (#105750)
Summary:
To save on binary size, some of the mobile configs don't include the
autograd kernels for built-in operators (VariableTypeEverything.cpp).
For the mobile build:
- we don't care about having a nice autograd fallback that warns if
an operator has incorrect autograd support. If you're running
a custom operator on mobile then it's already too late for us to warn
or error on it.
- for perf reasons, we do not want mobile to go through autograd_fallbac
for all operators (the boxing/unboxing adds overhead).
As a result, on mobile we set the fallback to the fallthrough.
Test Plan: existing tests and benchmarks
Differential Revision: D47674272
Pull Request resolved: https://github.com/pytorch/pytorch/pull/105750
Approved by: https://github.com/soulitzer
