Fix gcc v11 build failure RE: std::size_t

[matthewfeickert]

Following up Issue #136 and PR #137 given the build failure in https://its.cern.ch/jira/browse/ATLINFR-4147, it is seen that for gcc v11 there is a build failure of

[ 23%] Building CXX object CMakeFiles/lwtnn-stat.dir/src/FastInputPreprocessor.cxx.o
In file included from /__w/lwtnn/lwtnn/include/lwtnn/lightweight_network_config.hh:7,
                 from /__w/lwtnn/lwtnn/include/lwtnn/generic/FastInputPreprocessor.hh:4,
                 from /__w/lwtnn/lwtnn/src/FastInputPreprocessor.cxx:1:
/__w/lwtnn/lwtnn/include/lwtnn/NNLayerConfig.hh:69:17: error: 'size_t' was not declared in this scope; did you mean 'std::size_t'?
   69 |     std::vector<size_t> sources;
      |                 ^~~~~~
      |                 std::size_t
In file included from /usr/local/include/c++/11.1.0/bits/stl_algobase.h:59,
                 from /usr/local/include/c++/11.1.0/vector:60,
                 from /__w/lwtnn/lwtnn/include/lwtnn/NNLayerConfig.hh:11,
                 from /__w/lwtnn/lwtnn/include/lwtnn/lightweight_network_config.hh:7,
                 from /__w/lwtnn/lwtnn/include/lwtnn/generic/FastInputPreprocessor.hh:4,
                 from /__w/lwtnn/lwtnn/src/FastInputPreprocessor.cxx:1:
/usr/local/include/c++/11.1.0/x86_64-linux-gnu/bits/c++config.h:280:33: note: 'std::size_t' declared here
  280 |   typedef __SIZE_TYPE__         size_t;
      |                                 ^~~~~~
In file included from /__w/lwtnn/lwtnn/include/lwtnn/lightweight_network_config.hh:7,
                 from /__w/lwtnn/lwtnn/include/lwtnn/generic/FastInputPreprocessor.hh:4,
                 from /__w/lwtnn/lwtnn/src/FastInputPreprocessor.cxx:1:
/__w/lwtnn/lwtnn/include/lwtnn/NNLayerConfig.hh:69:23: error: template argument 1 is invalid
   69 |     std::vector<size_t> sources;
      |                       ^
/__w/lwtnn/lwtnn/include/lwtnn/NNLayerConfig.hh:69:23: error: template argument 2 is invalid
[ 26%] Building CXX object CMakeFiles/lwtnn.dir/src/FastInputPreprocessor.cxx.o
In file included from /__w/lwtnn/lwtnn/include/lwtnn/lightweight_network_config.hh:7,
                 from /__w/lwtnn/lwtnn/include/lwtnn/generic/FastInputPreprocessor.hh:4,
                 from /__w/lwtnn/lwtnn/src/FastInputPreprocessor.cxx:1:
/__w/lwtnn/lwtnn/include/lwtnn/NNLayerConfig.hh:69:17: error: 'size_t' was not declared in this scope; did you mean 'std::size_t'?
   69 |     std::vector<size_t> sources;
      |                 ^~~~~~
      |                 std::size_t
In file included from /usr/local/include/c++/11.1.0/bits/stl_algobase.h:59,
                 from /usr/local/include/c++/11.1.0/vector:60,
                 from /__w/lwtnn/lwtnn/include/lwtnn/NNLayerConfig.hh:11,
                 from /__w/lwtnn/lwtnn/include/lwtnn/lightweight_network_config.hh:7,
                 from /__w/lwtnn/lwtnn/include/lwtnn/generic/FastInputPreprocessor.hh:4,
                 from /__w/lwtnn/lwtnn/src/FastInputPreprocessor.cxx:1:
/usr/local/include/c++/11.1.0/x86_64-linux-gnu/bits/c++config.h:280:33: note: 'std::size_t' declared here
  280 |   typedef __SIZE_TYPE__         size_t;
      |                                 ^~~~~~
In file included from /__w/lwtnn/lwtnn/include/lwtnn/lightweight_network_config.hh:7,
                 from /__w/lwtnn/lwtnn/include/lwtnn/generic/FastInputPreprocessor.hh:4,
                 from /__w/lwtnn/lwtnn/src/FastInputPreprocessor.cxx:1:
/__w/lwtnn/lwtnn/include/lwtnn/NNLayerConfig.hh:69:23: error: template argument 1 is invalid
   69 |     std::vector<size_t> sources;
      |                       ^
/__w/lwtnn/lwtnn/include/lwtnn/NNLayerConfig.hh:69:23: error: template argument 2 is invalid
gmake[2]: *** [CMakeFiles/lwtnn-stat.dir/build.make:134: CMakeFiles/lwtnn-stat.dir/src/FastInputPreprocessor.cxx.o] Error 1
gmake[1]: *** [CMakeFiles/Makefile2:276: CMakeFiles/lwtnn-stat.dir/all] Error 2
gmake[1]: *** Waiting for unfinished jobs....
[ 28%] Building CXX object CMakeFiles/lwtnn.dir/src/FastGraph.cxx.o
In file included from /__w/lwtnn/lwtnn/include/lwtnn/lightweight_network_config.hh:7,
                 from /__w/lwtnn/lwtnn/src/FastGraph.cxx:1:
/__w/lwtnn/lwtnn/include/lwtnn/NNLayerConfig.hh:69:17: error: 'size_t' was not declared in this scope; did you mean 'std::size_t'?
   69 |     std::vector<size_t> sources;
      |                 ^~~~~~
      |                 std::size_t
In file included from /usr/local/include/c++/11.1.0/bits/stl_algobase.h:59,
                 from /usr/local/include/c++/11.1.0/vector:60,
                 from /__w/lwtnn/lwtnn/include/lwtnn/NNLayerConfig.hh:11,
                 from /__w/lwtnn/lwtnn/include/lwtnn/lightweight_network_config.hh:7,
                 from /__w/lwtnn/lwtnn/src/FastGraph.cxx:1:
/usr/local/include/c++/11.1.0/x86_64-linux-gnu/bits/c++config.h:280:33: note: 'std::size_t' declared here
  280 |   typedef __SIZE_TYPE__         size_t;
      |                                 ^~~~~~
In file included from /__w/lwtnn/lwtnn/include/lwtnn/lightweight_network_config.hh:7,
                 from /__w/lwtnn/lwtnn/src/FastGraph.cxx:1:
/__w/lwtnn/lwtnn/include/lwtnn/NNLayerConfig.hh:69:23: error: template argument 1 is invalid
   69 |     std::vector<size_t> sources;
      |                       ^
/__w/lwtnn/lwtnn/include/lwtnn/NNLayerConfig.hh:69:23: error: template argument 2 is invalid
gmake[2]: *** [CMakeFiles/lwtnn.dir/build.make:147: CMakeFiles/lwtnn.dir/src/FastGraph.cxx.o] Error 1
gmake[2]: *** Waiting for unfinished jobs....
gmake[2]: *** [CMakeFiles/lwtnn.dir/build.make:134: CMakeFiles/lwtnn.dir/src/FastInputPreprocessor.cxx.o] Error 1
gmake[1]: *** [CMakeFiles/Makefile2:1140: CMakeFiles/lwtnn.dir/all] Error 2
gmake: *** [Makefile:160: all] Error 2

where it looks like a relevant code block is

lwtnn/include/lwtnn/NNLayerConfig.hh

Lines 62 to 72 in dfdcdd4

	// graph node configuration
	struct NodeConfig
	{
	enum class Type {
	INPUT, INPUT_SEQUENCE, FEED_FORWARD, CONCATENATE, SEQUENCE,
	TIME_DISTRIBUTED, SUM };
	Type type;
	std::vector<size_t> sources;
	int index; // input node size, or layer number
	};
	}

cc @dguest