Remove dependency of backend CPU and fix CI

• Remove Backend CPU from CMakeLists.txt, setup.py and CI
• Add include shared script in build CI
• Add empty python requirements.txt for CI
• Generate random matrix in tests instead of loading an image from a path for CI on OVH
• Update Test to comply with opencv::convertNonZero that requires single channel array in v4.2 (https://docs.opencv.org/4.2.0/d2/de8/group__core__array.html#gaa4b89393263bb4d604e0fe5986723914)

.gitlab/ci/build.gitlab-ci.yml

+include:
+  - remote: 'https://gitlab.eclipse.org/eclipse/aidge/gitlab_shared_files/-/raw/main/.gitlab/ci/shared_script.gitlab-ci.yml'
+
 build:ubuntu_cpp:
   stage: build
   needs: []
@@ -8,10 +11,7 @@ build:ubuntu_cpp:
     - DEPENDENCY_JOB="build:ubuntu_cpp"
     # aidge_core
     - DEPENDENCY_NAME="aidge_core" 
-    - !reference [.download_dependency, script]   
-    # aidge_backend_cpu
-    - DEPENDENCY_NAME="aidge_backend_cpu"
-    - !reference [.download_dependency, script]    
+    - !reference [.download_dependency, script]       
   
     # Build current module
     - export CMAKE_PREFIX_PATH=../install_cpp
@@ -37,9 +37,6 @@ build:ubuntu_cpp_g++10:
     # aidge_core
     - DEPENDENCY_NAME="aidge_core" 
     - !reference [.download_dependency, script]   
-    # aidge_backend_cpu
-    - DEPENDENCY_NAME="aidge_backend_cpu"
-    - !reference [.download_dependency, script]    
 
     # Build current module
     - export CMAKE_PREFIX_PATH=../install_cpp
@@ -62,10 +59,7 @@ build:ubuntu_cpp_g++12:
     - DEPENDENCY_JOB="build:ubuntu_cpp"
     # aidge_core
     - DEPENDENCY_NAME="aidge_core" 
-    - !reference [.download_dependency, script]   
-    # aidge_backend_cpu
-    - DEPENDENCY_NAME="aidge_backend_cpu"
-    - !reference [.download_dependency, script]    
+    - !reference [.download_dependency, script]      
 
     # Build current module
     - export CMAKE_PREFIX_PATH=../install_cpp
@@ -88,9 +82,6 @@ build:ubuntu_cpp_clang12:
     - DEPENDENCY_JOB="build:ubuntu_cpp"
     # aidge_core
     - DEPENDENCY_NAME="aidge_core" 
-    - !reference [.download_dependency, script]   
-    # aidge_backend_cpu
-    - DEPENDENCY_NAME="aidge_backend_cpu"
     - !reference [.download_dependency, script]    
 
     # Build current module
@@ -114,10 +105,7 @@ build:ubuntu_cpp_clang15:
     - DEPENDENCY_JOB="build:ubuntu_cpp"
     # aidge_core
     - DEPENDENCY_NAME="aidge_core" 
-    - !reference [.download_dependency, script]   
-    # aidge_backend_cpu
-    - DEPENDENCY_NAME="aidge_backend_cpu"
-    - !reference [.download_dependency, script]    
+    - !reference [.download_dependency, script]     
 
     # Build current module
     - export CMAKE_PREFIX_PATH=../install_cpp
@@ -140,9 +128,6 @@ build:ubuntu_python:
     - DEPENDENCY_JOB="build:ubuntu_python"
     # aidge_core
     - DEPENDENCY_NAME="aidge_core" 
-    - !reference [.download_dependency, script]   
-    # aidge_backend_cpu
-    - DEPENDENCY_NAME="aidge_backend_cpu"
     - !reference [.download_dependency, script]    
 
     - python3 -m pip install virtualenv