python3Packages.pytorch: fixup builds with MKL

- Pass `blas.provider` into `buildInputs`, so that CMake can find the actual
  `mkl` for inspection of its cmake files and headers.

- Add `USE_MKL` correctly when the blas provider is `mkl`.

- Use the MKLDNN and MKLDNN_CBLAS flags by default, since `mkldnn` is FOSS and
  always available..

- Remove a patch for MKL 2019, since we've moved to 2020.

- Add a pythonImportsCheck for "torch" as a basic sanity-check

- Removed some unused variables at the top of the file

pkgs/development/python-modules/pytorch/default.nix

@@ -1,11 +1,10 @@
-{ stdenv, fetchurl, fetchgit, fetchpatch, buildPythonPackage, python, pythonOlder,
+{ stdenv, lib, fetchFromGitHub, fetchpatch, buildPythonPackage, python,
   cudaSupport ? false, cudatoolkit ? null, cudnn ? null, nccl ? null, magma ? null,
   mklDnnSupport ? true, useSystemNccl ? true,
   openMPISupport ? false, openmpi ? null,
-  buildBinaries ? false,
   buildDocs ? false,
   cudaArchList ? null,
-  fetchFromGitHub, lib, numpy, pyyaml, cffi, click, typing, cmake, hypothesis, numactl, psutil,
+  numpy, pyyaml, cffi, click, typing, cmake, dnnl, hypothesis, numactl, psutil,
   linkFarm, symlinkJoin,
 
   # virtual pkg that consistently instantiates blas across nixpkgs
@@ -152,7 +151,15 @@ in buildPythonPackage rec {
 
   BUILD_NAMEDTENSOR = true;
   BUILD_DOCS = buildDocs;
+
+  USE_MKL = blas.implementation == "mkl";
+
+  # Unlike MKL, MKLDNN is FOSS, so we enable support for it by default. Note
+  # that this was renamed to dnnl and then renamed again to oneDNN upstream, but
+  # pytorch still calls it by the old name mkldnn.
   USE_MKLDNN = mklDnnSupport;
+  USE_MKLDNN_CBLAS = mklDnnSupport;
+
   preBuild = ''
     export MAX_JOBS=$NIX_BUILD_CORES
     ${python.interpreter} setup.py build --cmake-only
@@ -174,7 +181,6 @@ in buildPythonPackage rec {
     done
   '';
 
-
   # Override the (weirdly) wrong version set by default. See
   # https://github.com/NixOS/nixpkgs/pull/52437#issuecomment-449718038
   # https://github.com/pytorch/pytorch/blob/v1.0.0/setup.py#L267
@@ -199,7 +205,7 @@ in buildPythonPackage rec {
     ninja
   ] ++ lib.optionals cudaSupport [ cudatoolkit_joined ];
 
-  buildInputs = [ blas ]
+  buildInputs = [ blas blas.provider dnnl ]
     ++ lib.optionals cudaSupport [ cudnn magma nccl ]
     ++ lib.optionals stdenv.isLinux [ numactl ];
 
@@ -214,10 +220,13 @@ in buildPythonPackage rec {
 
   checkInputs = [ hypothesis ninja psutil ];
 
-  doCheck = false; # tests take a long time for channel release, so doCheck should be overridden only when developing
+  # Tests take a long time and may be flaky, so just sanity-check imports
+  doCheck = false;
+  pythonImportsCheck = [
+    "torch"
+  ];
+
   checkPhase = with lib.versions; with lib.strings; concatStringsSep " " [
-    # MKL 2019.5-only workaround. See: https://github.com/NixOS/nixpkgs/issues/75611
-    (optionalString (blas.implementation == "mkl" && majorMinor blas.version == "2019.5") "KMP_INIT_AT_FORK=FALSE ")
     cudaStubEnv
     "${python.interpreter} test/run_test.py"
     "--exclude"