Bump the torch pin

It's requested by torchchat to have a newer version of torch nightly. Bump it from 4/15 to 4/21.

Author: iseeyuan

install_requirements.sh

@@ -61,7 +61,7 @@ done
 # NOTE: If a newly-fetched version of the executorch repo changes the value of
 # NIGHTLY_VERSION, you should re-run this script to install the necessary
 # package versions.
-NIGHTLY_VERSION=dev20240415
+NIGHTLY_VERSION=dev20240421
 
 # The pip repository that hosts nightly torch packages.
 TORCH_NIGHTLY_URL="https://download.pytorch.org/whl/nightly/cpu"