[BE] Make Eigen an optional dependency (#155955)

Whose version is controlled by `eigen_pin.txt`, but which will be installed only if BLAS providers could not be found.
Why this is good for CI: we don't really build with Eigen ever and gitlab can be down when github is up, which causes spurious CI failures in the past, for example.

Remove eigen submodule and replace it with eigen_pin.txt

Fixes https://github.com/pytorch/pytorch/issues/108773
Pull Request resolved: https://github.com/pytorch/pytorch/pull/155955
Approved by: https://github.com/atalman
ghstack dependencies: #155947, #155954

.gitmodules

@@ -2,10 +2,6 @@
     ignore = dirty
     path = third_party/pybind11
     url = https://github.com/pybind/pybind11.git
-[submodule "third_party/eigen"]
-    ignore = dirty
-    path = third_party/eigen
-    url = https://gitlab.com/libeigen/eigen.git
 [submodule "third_party/googletest"]
     ignore = dirty
     path = third_party/googletest

cmake/Dependencies.cmake

@@ -778,25 +778,6 @@ elseif(NOT TARGET fp16 AND USE_SYSTEM_FP16)
 endif()
 list(APPEND Caffe2_DEPENDENCY_LIBS fp16)
 
-# ---[ EIGEN
-# Due to license considerations, we will only use the MPL2 parts of Eigen.
-set(EIGEN_MPL2_ONLY 1)
-if(USE_SYSTEM_EIGEN_INSTALL)
-  find_package(Eigen3)
-  if(EIGEN3_FOUND)
-    message(STATUS "Found system Eigen at " ${EIGEN3_INCLUDE_DIR})
-  else()
-    message(STATUS "Did not find system Eigen. Using third party subdirectory.")
-    set(EIGEN3_INCLUDE_DIR ${CMAKE_CURRENT_LIST_DIR}/../third_party/eigen)
-    caffe2_update_option(USE_SYSTEM_EIGEN_INSTALL OFF)
-  endif()
-else()
-  message(STATUS "Using third party subdirectory Eigen.")
-  set(EIGEN3_INCLUDE_DIR ${CMAKE_CURRENT_LIST_DIR}/../third_party/eigen)
-endif()
-include_directories(SYSTEM ${EIGEN3_INCLUDE_DIR})
-
-
 # ---[ Python Interpreter
 # If not given a Python installation, then use the current active Python
 if(NOT Python_EXECUTABLE)
@@ -810,6 +791,29 @@ if(NOT Python_EXECUTABLE)
   endif()
 endif()
 
+
+# ---[ EIGEN
+# Due to license considerations, we will only use the MPL2 parts of Eigen.
+set(EIGEN_MPL2_ONLY 1)
+if(USE_SYSTEM_EIGEN_INSTALL)
+  find_package(Eigen3)
+  if(EIGEN3_FOUND)
+    message(STATUS "Found system Eigen at " ${EIGEN3_INCLUDE_DIR})
+  else()
+    message(STATUS "Did not find system Eigen. Using third party subdirectory.")
+    execute_process(COMMAND ${Python_EXECUTABLE} ../tools/optional_modules.py checkout_eigen
+                    WORKING_DIRECTORY ${CMAKE_CURRENT_LIST_DIR})
+
+    set(EIGEN3_INCLUDE_DIR ${CMAKE_CURRENT_LIST_DIR}/../third_party/eigen)
+    caffe2_update_option(USE_SYSTEM_EIGEN_INSTALL OFF)
+  endif()
+else()
+  message(STATUS "Using third party subdirectory Eigen.")
+  set(EIGEN3_INCLUDE_DIR ${CMAKE_CURRENT_LIST_DIR}/../third_party/eigen)
+endif()
+include_directories(SYSTEM ${EIGEN3_INCLUDE_DIR})
+
+
 if(BUILD_PYTHON)
   set(PYTHON_COMPONENTS Development.Module)
   if(USE_NUMPY)

third_party/eigen_pin.txt

@@ -0,0 +1 @@
+3.4.0

tools/optional_submodules.py

@@ -43,5 +43,21 @@ def checkout_nccl() -> None:
         _checkout_by_tag("https://github.com/NVIDIA/nccl", release_tag)
 
 
+def checkout_eigen() -> None:
+    eigen_tag = _read_file(third_party_path / "eigen_pin.txt")
+    print(f"-- Checkout Eigen release tag: {eigen_tag}")
+    eigen_basedir = third_party_path / "eigen"
+    if not eigen_basedir.exists():
+        _checkout_by_tag("https://gitlab.com/libeigen/eigen", eigen_tag)
+
+
 if __name__ == "__main__":
-    checkout_nccl()
+    import sys
+
+    if len(sys.argv) == 1:
+        # If no arguments are given checkout all optional dependency
+        checkout_nccl()
+        checkout_eigen()
+    else:
+        # Otherwise just call top-level function of choice
+        globals()[sys.argv[1]]()