Not explicitly set the manifest filename in Windows (#91988)

I'm at a loss to explain why this happens, but not setting the manifest file explicitly in the linker fixes it.

### Testing locally

* With `/MANIFESTFILE:bin\torch_python.dll.manifest`
```
C:\PROGRA~2\MICROS~2\2019\BUILDT~1\VC\Tools\MSVC\1428~1.293\bin\Hostx64\x64\link.exe /nologo @CMakeFiles\torch_python.rsp /out:bin\torch_python.dll /implib:lib\torch_python.lib /pdb:bin\torch_python.pdb /dll /version:0.0 /machine:x64 /ignore:4049 /ignore:4217 /ignore:4099 /INCREMENTAL:NO /NODEFAULTLIB:LIBCMT.LIB -WHOLEARCHIVE:C:/actions-runner/_work/pytorch/pytorch/build/lib/onnx.lib /MANIFEST /MANIFESTFILE:bin\torch_python.dll.manifest

LINK : fatal error LNK1000: Internal error during CImplib::EmitImportThunk
```

* Work fine without the flag
```
C:\PROGRA~2\MICROS~2\2019\BUILDT~1\VC\Tools\MSVC\1428~1.293\bin\Hostx64\x64\link.exe /nologo @CMakeFiles\torch_python.rsp /out:bin\torch_python.dll /implib:lib\torch_python.lib /pdb:bin\torch_python.pdb /dll /version:0.0 /machine:x64 /ignore:4049 /ignore:4217 /ignore:4099 /INCREMENTAL:NO /NODEFAULTLIB:LIBCMT.LIB -WHOLEARCHIVE:C:/actions-runner/_work/pytorch/pytorch/build/lib/onnx.lib /MANIFEST
```

In both case, the `/MANIFEST` flag is set, so the manifest file is there.  In the latter case, the filename comes by appending `.manifest` suffix to `bin\torch_python.dll`.  Thus, it's still correctly be `bin\torch_python.dll.manifest`.  Weird.

```
C:\actions-runner\_work\pytorch\pytorch>ls -la build/bin/torch_*
-rwxr-xr-x 1 runneruser 197121 246796288 Jan 11 04:30 build/bin/torch_cpu.dll
-rw-r--r-- 1 runneruser 197121       381 Jan 11 04:26 build/bin/torch_cpu.dll.manifest
-rwxr-xr-x 1 runneruser 197121      9728 Jan 11 03:55 build/bin/torch_global_deps.dll
-rw-r--r-- 1 runneruser 197121       381 Jan 11 03:55 build/bin/torch_global_deps.dll.manifest
-rwxr-xr-x 1 runneruser 197121  11746816 Jan 11 04:31 build/bin/torch_python.dll
-rw-r--r-- 1 runneruser 197121       381 Jan 11 04:30 build/bin/torch_python.dll.manifest
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/91988
Approved by: https://github.com/malfet, https://github.com/Blackhex, https://github.com/ZainRizvi

.github/workflows/pull.yml

@@ -230,7 +230,6 @@ jobs:
       test-matrix: ${{ needs.linux-bionic-py3_7-clang8-xla-build.outputs.test-matrix }}
 
   win-vs2019-cpu-py3-build:
-    if: false
     name: win-vs2019-cpu-py3
     uses: ./.github/workflows/_win-build.yml
     with:
@@ -244,7 +243,6 @@ jobs:
         ]}
 
   win-vs2019-cpu-py3-test:
-    if: false
     name: win-vs2019-cpu-py3
     uses: ./.github/workflows/_win-test.yml
     needs: win-vs2019-cpu-py3-build

CMakeLists.txt

@@ -529,6 +529,16 @@ if(MSVC)
     string(APPEND ${flag_var} " /ignore:4049 /ignore:4217 /ignore:4099")
   endforeach(flag_var)
 
+  foreach(flag_var
+      CMAKE_SHARED_LINKER_FLAGS)
+    # https://github.com/pytorch/pytorch/issues/91933: Don't set the manifest filename
+    # explicitly helps fix the linker error when linking torch_python.dll. The manifest
+    # file would still be there in the correct format torch_python.dll.manifest
+    if(${flag_var} MATCHES "/MANIFESTFILE:.*\\.manifest")
+      string(REGEX REPLACE "/MANIFESTFILE:.*\\.manifest" "" ${flag_var} "${${flag_var}}")
+    endif()
+  endforeach(flag_var)
+
   # Try harder
   string(APPEND CMAKE_CUDA_FLAGS " -Xcompiler /w -w")
 

c10/util/env.h

@@ -14,7 +14,14 @@ namespace utils {
 // NB:
 // Issues a warning if the value of the environment variable is not 0 or 1.
 inline optional<bool> check_env(const char* name) {
+#ifdef _MSC_VER
+#pragma warning(push)
+#pragma warning(disable : 4996)
+#endif
   auto envar = std::getenv(name);
+#ifdef _MSC_VER
+#pragma warning(pop)
+#endif
   if (envar) {
     if (strcmp(envar, "0") == 0) {
       return false;