Revert "Add Differentiable Physics: Mass-Spring System example" (pytorch#1360)

Revert "Add Differentiable Physics: Mass-Spring System example (pytorch#1359)"

This reverts commit 7c35995.

Author: msaroufim

differentiable_physics/mass_spring.py

@@ -170,11 +170,6 @@ function gat() {
   uv run main.py --epochs 1 --dry-run || error "graph attention network failed"
 }
 
-function differentiable_physics() {
-  uv run mass_spring.py --mode train --epochs 5 --steps 3 || error "differentiable_physics example failed"
-}
-
-
 eval "base_$(declare -f stop)"
 
 function stop() {
@@ -228,7 +223,6 @@ function run_all() {
   run fx
   run gcn
   run gat
-  run differentiable_physics
 }
 
 # by default, run all examples