[aten] Make aten::flatten call native::reshape (#50859)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/50859
Test Plan:
Unit test:
```
buck test //caffe2/test:torch
```
Benchmark:
```
MKL_NUM_THREADS=1 OMP_NUM_THREADS=1 numactl -m 0 -C 13 \
./buck-out/opt/gen/caffe2/caffe2/fb/predictor/ptvsc2_predictor_bench \
--scripted_model=/home/hlu/ads/adindexer/adindexer_ctr_mobilefeed/pt/merge_v2/traced_precomputation.pt \
--pt_inputs=/home/hlu/ads/adindexer/adindexer_ctr_mobilefeed/pt/merge_v2/container_precomputation_bs20.pt \
--iters=10000 --warmup_iters=10000 --num_threads=1 --pt_enable_static_runtime=true \
--pt_cleanup_activations=true --pt_enable_out_variant=true --do_profile=true
```
Reduces the total time spent on flatten from 1.22% to 0.97% (net 0.25% reduction).
```
Before:
Static runtime ms per iter: 0.0725054. Iters per second: 13792.1
0.000857179 ms. 1.21862%. aten::flatten (1 nodes)
After:
Static runtime ms per iter: 0.0720371. Iters per second: 13881.7
0.000686155 ms. 0.97151%. aten::flatten (1 nodes)
```
Reviewed By: ajyu
Differential Revision: D25986759
fbshipit-source-id: dc0f542c56a688d331d349845b78084577970476
