Remove strict dependency on a specific PyTorch version (#135)

Summary:
When building artifacts, PyTorch version is set strictly (`pytorch=xxx`) to the current version used in the build image. This forces reinstall of pytorch when installing csprng, although there is no actual reasons to use a specific version.

Opacus is affected and blocked because of this problem: meta-pytorch/opacus#351

This PR changes deps in setup.py to just `pytorch` (without specifying bounds).

Pull Request resolved: #135

Reviewed By: ffuuugor

Differential Revision: D35408862

Pulled By: romovpa

fbshipit-source-id: 30c18562e49ae439a31a005507b397713f40fd36

Author: Peter Romov

setup.py

@@ -51,12 +51,9 @@ def write_version_file():
 with open("README.md", "r") as fh:
     long_description = fh.read()
 
-pytorch_dep = "torch"
-if os.getenv("PYTORCH_VERSION"):
-    pytorch_dep += "==" + os.getenv("PYTORCH_VERSION")
 
 requirements = [
-    pytorch_dep,
+    "torch",
 ]