Restructuring C++ project: (#3146)

Summary:
* Reduce unnecessary header inclusions in models and io.

* Move autocast to separate folder and hide autograd implementation in an anonymous namespace.

* Moving files in subfolders.

Reviewed By: fmassa

Differential Revision: D25461523

fbshipit-source-id: 756eeb6848aacaa474de4825ed4c1045d17e2cea

torchvision/csrc/cpu/image/read_image_cpu.cpp → torchvision/csrc/io/image/cpu/read_image_cpu.cpp

 #include "read_image_cpu.h"
+
 #include "readjpeg_cpu.h"
 #include "readpng_cpu.h"
 

torchvision/csrc/cpu/image/read_image_cpu.h → torchvision/csrc/io/image/cpu/read_image_cpu.h

 #pragma once
 
-#include <torch/torch.h>
-#include "image_read_mode.h"
+#include <torch/types.h>
+#include "../image_read_mode.h"
 
 C10_EXPORT torch::Tensor decode_image(
     const torch::Tensor& data,

torchvision/csrc/cpu/image/read_write_file_cpu.h → torchvision/csrc/io/image/cpu/read_write_file_cpu.h

 #pragma once
 
-#include <errno.h>
 #include <sys/stat.h>
-#include <torch/torch.h>
+#include <torch/types.h>
 
 C10_EXPORT torch::Tensor read_file(const std::string& filename);