[DevX] Add tool and doc on partial debug builds

[malfet]

Turned command sequence mentioned in https://dev-discuss.pytorch.org/t/how-to-get-a-fast-debug-build/1597 and in various discussions into a tool that I use almost daily to debug crashes or correctness issues in the codebase

Essentially it allows one to turn this:

Process 87729 stopped
* thread #1, queue = 'com.apple.main-thread', stop reason = breakpoint 1.1
    frame #0: 0x00000001023d55a8 libtorch_python.dylib`at::indexing::impl::applySelect(at::Tensor const&, long long, c10::SymInt, long long, c10::Device const&, std::__1::optional<c10::ArrayRef<c10::SymInt>> const&)
libtorch_python.dylib`at::indexing::impl::applySelect:
->  0x1023d55a8 <+0>:  sub    sp, sp, #0xd0
    0x1023d55ac <+4>:  stp    x24, x23, [sp, #0x90]
    0x1023d55b0 <+8>:  stp    x22, x21, [sp, #0xa0]
    0x1023d55b4 <+12>: stp    x20, x19, [sp, #0xb0]

into this

Process 87741 stopped
* thread #1, queue = 'com.apple.main-thread', stop reason = breakpoint 1.1
    frame #0: 0x00000001024e2628 libtorch_python.dylib`at::indexing::impl::applySelect(self=0x00000001004ee8a8, dim=0, index=(data_ = 3), real_dim=0, (null)=0x000000016fdfe535, self_sizes= Has Value=true ) at TensorIndexing.h:239:7
   236 	    const at::Device& /*self_device*/,
   237 	    const c10::optional<SymIntArrayRef>& self_sizes) {
   238 	  // See NOTE [nested tensor size for indexing]
-> 239 	  if (self_sizes.has_value()) {
   240 	    auto maybe_index = index.maybe_as_int();
   241 	    if (maybe_index.has_value()) {
   242 	      TORCH_CHECK_INDEX(

while retaining good performance for the rest of the codebase

[pytorch-bot]

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✅ You can merge normally! (1 Unrelated Failure)

As of commit bfe0163 with merge base 87da0e1 ():

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / linux-focal-cuda12.1-py3.10-gcc9-sm86 / test (default, 1, 5, linux.g5.4xlarge.nvidia.gpu) (gh)
  dynamo/test_dynamic_shapes.py::DynamicShapesCtxManagerTests::test_cuda_event_method_dynamic_shapes

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[atalman]

LGTM, but lint needs to be fixed

[malfet]

@pytorchbot merge -f "lint is green"

[pytorchmergebot]

Merge started

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[albanD]

I'm curious what you think is the UX tradeoff between a separate script vs using an env variable + the usual setup.py develop ?