Set make max load when building libtorch (pytorch#89237)

The nccl build is still OOM sometimes when using `$(MAKE)`:

```
virtual memory exhausted: Cannot allocate memory
Makefile:73: recipe for target '/var/lib/jenkins/cpp-build/caffe2/build/nccl/obj/collectives/device/devlink.o' failed
make[5]: *** [/var/lib/jenkins/cpp-build/caffe2/build/nccl/obj/collectives/device/devlink.o] Error 1
make[5]: Leaving directory '/var/lib/jenkins/workspace/third_party/nccl/nccl/src/collectives/device'
```

* https://github.com/pytorch/pytorch/actions/runs/3476485191/jobs/5811758058
* https://github.com/pytorch/pytorch/actions/runs/3422228421/jobs/5702153639

So trying to set the same limit here as when building with ninja

Pull Request resolved: pytorch#89237
Approved by: https://github.com/malfet

cmake/External/nccl.cmake

@@ -15,23 +15,24 @@ if(NOT __NCCL_INCLUDED)
     # this second replacement is needed when there are multiple archs
     string(REPLACE ";-gencode" " -gencode" NVCC_GENCODE "${NVCC_GENCODE}")
 
-    if("${CMAKE_GENERATOR}" MATCHES "Make")
-      # Recursive make with jobserver for parallelism
-      set(MAKE_COMMAND "$(MAKE)")
+    if(DEFINED ENV{MAX_JOBS})
+      set(MAX_JOBS "$ENV{MAX_JOBS}")
     else()
-      if(DEFINED ENV{MAX_JOBS})
-        set(MAX_JOBS "$ENV{MAX_JOBS}")
-      else()
-        include(ProcessorCount)
-        ProcessorCount(NUM_HARDWARE_THREADS)
-        # Assume 2 hardware threads per cpu core
-        math(EXPR MAX_JOBS "${NUM_HARDWARE_THREADS} / 2")
-        # ProcessorCount might return 0, set to a positive number
-        if(MAX_JOBS LESS 2)
-            set(MAX_JOBS 2)
-        endif()
+      include(ProcessorCount)
+      ProcessorCount(NUM_HARDWARE_THREADS)
+      # Assume 2 hardware threads per cpu core
+      math(EXPR MAX_JOBS "${NUM_HARDWARE_THREADS} / 2")
+      # ProcessorCount might return 0, set to a positive number
+      if(MAX_JOBS LESS 2)
+        set(MAX_JOBS 2)
       endif()
+    endif()
 
+    if("${CMAKE_GENERATOR}" MATCHES "Make")
+      # Recursive make with jobserver for parallelism, and also put a load limit
+      # here to avoid flaky OOM, https://www.gnu.org/software/make/manual/html_node/Parallel.html
+      set(MAKE_COMMAND "$(MAKE)" "-l${MAX_JOBS}")
+    else()
       # Parallel build with CPU load limit to avoid oversubscription
       set(MAKE_COMMAND "make" "-j${MAX_JOBS}" "-l${MAX_JOBS}")
     endif()