[seq2seq] document the caveat of leaky native amp (#8930)

* document the caveat of leaky native amp

* Update examples/seq2seq/README.md
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

examples/seq2seq/README.md

@@ -79,6 +79,11 @@ test.target
 ```
 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
+
+
 ### Tips and Tricks
 
 General Tips:
@@ -592,4 +597,3 @@ The feature is still experimental, because:
 + we can make it much more robust if we have memory mapped/preprocessed datasets.
 + The speedup over sortish sampler is not that large at the moment.
 
-