native amp leak fix landed in 1.7.1 (#9115)

update README with good news that the leak fix has been applied to pytorch-1.7.1.

examples/seq2seq/README.md

@@ -97,7 +97,7 @@ The `.source` files are the input, the `.target` files are the desired output.
 
 ### Potential issues
 
-- native AMP (`--fp16` and no apex) may lead to a huge memory leak and require 10x gpu memory. This has been fixed in pytorch-nightly and the minimal official version to have this fix will be pytorch-1.8. Until then if you have to use mixed precision please use AMP only with pytorch-nightly or NVIDIA's apex. Reference: https://github.com/huggingface/transformers/issues/8403
+- native AMP (`--fp16` and no apex) may lead to a huge memory leak and require 10x gpu memory. This has been fixed in pytorch-nightly and the minimal official version to have this fix will be pytorch-1.7.1. Until then if you have to use mixed precision please use AMP only with pytorch-nightly or NVIDIA's apex. Reference: https://github.com/huggingface/transformers/issues/8403
 
 
 ### Tips and Tricks