Introduce the torchrun entrypoint (#64049)

Summary:
This PR introduces a new `torchrun` entrypoint that simply "points" to `python -m torch.distributed.run`. It is shorter and less error-prone to type and gives a nicer syntax than a rather cryptic `python -m ...` command line. Along with the new entrypoint the documentation is also updated and places where `torch.distributed.run` are mentioned are replaced with `torchrun`.

cc pietern mrshenli pritamdamania87 zhaojuanmao satgera rohan-varma gqchen aazzolini osalpekar jiayisuse agolynski SciPioneer H-Huang mrzzd cbalioglu gcramer23

Pull Request resolved: https://github.com/pytorch/pytorch/pull/64049

Reviewed By: cbalioglu

Differential Revision: D30584041

Pulled By: kiukchung

fbshipit-source-id: d99db3b5d12e7bf9676bab70e680d4b88031ae2d
diff --git a/setup.py b/setup.py
index a200982..6d9ed53 100644
--- a/setup.py
+++ b/setup.py
@@ -854,6 +854,7 @@
         'console_scripts': [
             'convert-caffe2-to-onnx = caffe2.python.onnx.bin.conversion:caffe2_to_onnx',
             'convert-onnx-to-caffe2 = caffe2.python.onnx.bin.conversion:onnx_to_caffe2',
+            'torchrun = torch.distributed.run:main',
         ]
     }