Upgrade - C++ 17 and gcc 7 minimum (#526)

Summary:
Move to C++ 17. Require gcc >= 7. Upgrade CI baselines to use gcc-7.

Pull Request resolved: flashlight/flashlight#526

Test Plan: Manual build and test + CI

Reviewed By: vineelpratap

Differential Revision: D27606016

Pulled By: jacobkahn

fbshipit-source-id: 5a88e087529ca9d0fa868292678012fc572668af

CMakeLists.txt

@@ -12,7 +12,7 @@ if(CCACHE_PROGRAM)
 endif()
 
 set(CMAKE_MODULE_PATH "${CMAKE_CURRENT_LIST_DIR}/cmake")
-set(CMAKE_CXX_STANDARD 14)
+set(CMAKE_CXX_STANDARD 17)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
 
 # Default directories for installation
@@ -93,6 +93,9 @@ set(FL_BUILD_BINARY_OUTPUT_DIR "${CMAKE_CURRENT_BINARY_DIR}/bin")
 # CUDA/nvcc setup
 if (FL_USE_CUDA)
   enable_language(CUDA)
+  # The CUDA standard is still C++14 to enable interopability with
+  # slightly older and still well-supported versions of CUDA/nvcc
+  # (e.g. CUDA < 11).
   set(CMAKE_CUDA_STANDARD 14)
   set(CMAKE_CUDA_STANDARD_REQUIRED ON)
   include(${CMAKE_MODULE_PATH}/CUDAUtils.cmake)

README.md

@@ -110,7 +110,7 @@ D.backward(); // populates A.grad() along with gradients for B, C, and D.
 
 ### Requirements
 At minimum, compilation requires:
-- A C++ compiler with good C++14 support (e.g. gcc/g++ >= 5)
+- A C++ compiler with good C++17 support (e.g. gcc/g++ >= 7)
 - [CMake](https://cmake.org/) — version 3.10 or later, and ``make``
 - A Linux-based operating system.
 
@@ -432,7 +432,7 @@ The following CMake configuration links Flashlight and sets include directories:
 
 ```cmake
 cmake_minimum_required(VERSION 3.10)
-set(CMAKE_CXX_STANDARD 14)
+set(CMAKE_CXX_STANDARD 17)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
 
 add_executable(myProject project.cpp)