use `const auto&` as type for grad alias (#63949)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/63949

This is an extension of the discussion in
https://github.com/pytorch/pytorch/pull/63040#discussion_r687793027.

Test Plan: Imported from OSS

Reviewed By: albanD

Differential Revision: D30546789

Pulled By: dagitses

fbshipit-source-id: 3046aff4f129d5492d73dfb67717a824e16ffee8

diff --git a/tools/autograd/gen_autograd_functions.py b/tools/autograd/gen_autograd_functions.py
index 7d852ad..08136ab 100644 (file)
--- a/tools/autograd/gen_autograd_functions.py
+++ b/tools/autograd/gen_autograd_functions.py
@@ -479,7 +479,7 @@ def process_function(info: DifferentiabilityInfo, template: CodeTemplate) -> str
     body: List[str] = []
 
     if uses_single_grad(info):
-        body.append('auto& grad = grads[0];')
+        body.append('const auto& grad = grads[0];')
 
     def emit_derivative(
         derivative: Derivative,