pin torchvision version to 0.15.2 in dev-requirements (#698)

Summary:
Pull Request resolved: #698

# Context
We are seeing failed unit test, caused by torchvision error
{F1452783880}
Torchvision recently updated to 0.17, which is when I suspect this issue occurred.
# This Diff
For now, pinning the version to the last 0.15.x one, as the failing unit test occurs in 0.16 or up

Reviewed By: gunchu

Differential Revision: D53456248

fbshipit-source-id: bcfaaf3404fa3c07abbe34a93d4c8760aa4a32d3

examples/mnist/requirements.txt

@@ -1,4 +1,4 @@
 torch
 torcheval
 torchtnt
-torchvision
+torchvision==0.15.2