Allow loading large model files

[theSoenke]

Loading large models > 2.5GB in Ruby with File.binread fails with an error:

binread': Invalid argument @ io_fread - ./model.pth (Errno::EINVAL)

This change should allow loading arbitrarily large models as long as enough memory is available.

[ankane]

Hi @theSoenke, thanks for the PR! I think it'd be most efficient to move reading the file to C++ (where it can be read directly into a std::vector<char>).

[theSoenke]

@ankane that sounds like a better idea. Loading these chunks in Ruby didn't feel quite right. Adapted the PR to use C++

Benckmark:

puts Benchmark.measure {
  10.times do
    Torch.load("./model.pth")
  end
}

cpp:
84.285722  36.134938 120.420660 (132.447201)

ruby:
54.531321  67.399236 121.930557 (222.239866)

CPU time seemed roughly similar in my tests but for the actual time it made a difference

[ankane]

Awesome, thanks @theSoenke!