Bump the torch pin (#3199)

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

Pull Request resolved: #3199

Reviewed By: malfet

Differential Revision: D56420105

Pulled By: iseeyuan

fbshipit-source-id: 3d2a9b0f8dbb48f0a81c7cdef8e419206b036faf

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"