gdb special command to print tensors (#54339)

Summary:
This is something which I wrote because it was useful during my debugging sessions, but I think it might be generally useful to other people as well so I took the liberty of proposing an official `pytorch-gdb` extension.

`pytorch-gdb` is a gdb script written in python. Currently, it contains only one command: `torch-tensor-repr`, which prints a human-readable repr of an `at::Tensor` object. Example:
```
Breakpoint 1, at::native::neg (self=...) at [...]/pytorch/aten/src/ATen/native/UnaryOps.cpp:520
520     Tensor neg(const Tensor& self) { return unary_op_impl(self, at::neg_out); }
(gdb) # the default repr of 'self' is not very useful
(gdb) p self
$1 = (const at::Tensor &) 0x7ffff72ed780: {impl_ = {target_ = 0x5555559df6e0}}
(gdb) torch-tensor-repr self
Python-level repr of self:
tensor([1., 2., 3., 4.], dtype=torch.float64)
```

The idea is that by having an official place where to put these things, `pytorch-gdb` will slowly grow other useful features and make the pytorch debugging experience nicer and faster.

Pull Request resolved: https://github.com/pytorch/pytorch/pull/54339

Reviewed By: bdhirsh

Differential Revision: D27253674

Pulled By: ezyang

fbshipit-source-id: dba219e126cc2fe66b2d26740f3a8e3b886e56f5