Update on "Improve and expose cpp_backtrace to python binding"

We can now get cpp stack trace by calling torch.utils.get_cpp_backtrace()

Sample output when calling from a torch_dispatch stack: 
```
<omitting python frames>
frame #23: torch::handle_torch_function_no_python_arg_parser(c10::ArrayRef<pybind11::handle>, _object*, _object*, char const*, _object*, char const*, torch::TorchFunctionName) (0x7f69330bab90 in /fsx/users/bahuang/repos/pytorch_fsx/torch/csrc/utils/python_arg_parser.cpp:323)
frame #24: <unknown function> (0x7f6932a09e79 in /fsx/users/bahuang/repos/pytorch_fsx/torch/csrc/autograd/python_variable.cpp:2252)
frame #25: <unknown function> (0x7f69261aee33 in /fsx/users/bahuang/repos/pytorch_fsx/aten/src/ATen/core/PythonFallbackKernel.cpp:56)
frame #26: <unknown function> (0x7f69261afef9 in /fsx/users/bahuang/repos/pytorch_fsx/aten/src/ATen/core/boxing/BoxedKernel_impl.h:19)
frame #27: c10::BoxedKernel::callBoxed(c10::OperatorHandle const&, c10::DispatchKeySet, std::vector<c10::IValue, std::allocator<c10::IValue> >*) const (0x7f6932aadced in /fsx/users/bahuang/repos/pytorch_fsx/aten/src/ATen/core/boxing/BoxedKernel_impl.h:41)
frame #28: <unknown function> (0x7f6926fae9b9 in /fsx/users/bahuang/repos/pytorch_fsx/aten/src/ATen/core/boxing/impl/boxing.h:227)
frame #29: at::Tensor c10::Dispatcher::redispatch<at::Tensor, at::Tensor const&>(c10::TypedOperatorHandle<at::Tensor (at::Tensor const&)> const&, c10::DispatchKeySet, at::Tensor const&) const (0x7f6926e821f5 in /fsx/users/bahuang/repos/pytorch_fsx/aten/src/ATen/core/boxing/KernelFunction_impl.h:106)
frame #30: at::_ops::alias::redispatch(c10::DispatchKeySet, at::Tensor const&) (0x7f6927142c31 in /fsx/users/bahuang/repos/pytorch_fsx/aten/src/ATen/core/dispatch/Dispatcher.h:438)
frame #31: <unknown function> (0x7f692ae4f8be in /fsx/users/bahuang/repos/pytorch_fsx/torch/csrc/autograd/generated/ADInplaceOrViewType_1.cpp:1361)
frame #32: <unknown function> (0x7f692ae4f9b1 in /fsx/users/bahuang/repos/pytorch_fsx/torch/csrc/autograd/generated/ADInplaceOrViewType_1.cpp:1362)
frame #33: <unknown function> (0x7f692aef77e9 in /fsx/users/bahuang/repos/pytorch_fsx/aten/src/ATen/core/boxing/impl/WrapFunctionIntoFunctor.h:13)
frame #34: <unknown function> (0x7f6926fae7d8 in /fsx/users/bahuang/repos/pytorch_fsx/aten/src/ATen/core/boxing/KernelFunction_impl.h:50)
frame #35: at::Tensor c10::Dispatcher::redispatch<at::Tensor, at::Tensor const&>(c10::TypedOperatorHandle<at::Tensor (at::Tensor const&)> const&, c10::DispatchKeySet, at::Tensor const&) const (0x7f6926e821c9 in /fsx/users/bahuang/repos/pytorch_fsx/aten/src/ATen/core/boxing/KernelFunction_impl.h:97)
frame #36: at::_ops::alias::redispatch(c10::DispatchKeySet, at::Tensor const&) (0x7f6927142c31 in /fsx/users/bahuang/repos/pytorch_fsx/aten/src/ATen/core/dispatch/Dispatcher.h:438)
frame #37: <unknown function> (0x7f6929ec654a in /fsx/users/bahuang/repos/pytorch_fsx/build/aten/src/ATen/RedispatchFunctions.h:10697)
frame #38: <unknown function> (0x7f6929d9edae in /fsx/users/bahuang/repos/pytorch_fsx/torch/csrc/autograd/generated/VariableType_1.cpp:2837)
frame #39: <unknown function> (0x7f6929d9f043 in /fsx/users/bahuang/repos/pytorch_fsx/torch/csrc/autograd/generated/VariableType_1.cpp:2838)
frame #40: <unknown function> (0x7f6929e7d2f9 in /fsx/users/bahuang/repos/pytorch_fsx/aten/src/ATen/core/boxing/impl/WrapFunctionIntoFunctor.h:13)
frame #41: <unknown function> (0x7f6929eb1344 in /fsx/users/bahuang/repos/pytorch_fsx/aten/src/ATen/core/boxing/impl/make_boxed_from_unboxed_functor.h:478)
frame #42: <unknown function> (0x7f6929ea7b99 in /fsx/users/bahuang/repos/pytorch_fsx/aten/src/ATen/core/boxing/impl/make_boxed_from_unboxed_functor.h:490)
frame #43: <unknown function> (0x7f6929e7d370 in /fsx/users/bahuang/repos/pytorch_fsx/aten/src/ATen/core/boxing/impl/make_boxed_from_unboxed_functor.h:563)
frame #44: <unknown function> (0x7f6929e7d43a in /fsx/users/bahuang/repos/pytorch_fsx/c10/util/C++17.h:239)
frame #45: <unknown function> (0x7f6929e7d48c in /fsx/users/bahuang/repos/pytorch_fsx/c10/util/C++17.h:364)
frame #46: <unknown function> (0x7f6929e7d50a in /fsx/users/bahuang/repos/pytorch_fsx/aten/src/ATen/core/boxing/impl/make_boxed_from_unboxed_functor.h:554)
frame #47: c10::BoxedKernel::callBoxed(c10::OperatorHandle const&, c10::DispatchKeySet, std::vector<c10::IValue, std::allocator<c10::IValue> >*) const (0x7f6932aadced in /fsx/users/bahuang/repos/pytorch_fsx/aten/src/ATen/core/boxing/BoxedKernel_impl.h:41)
frame #48: c10::KernelFunction::callBoxed(c10::OperatorHandle const&, c10::DispatchKeySet, std::vector<c10::IValue, std::allocator<c10::IValue> >*) const (0x7f6932aadd26 in /fsx/users/bahuang/repos/pytorch_fsx/aten/src/ATen/core/boxing/KernelFunction_impl.h:43)
frame #49: c10::Dispatcher::redispatchBoxed(c10::OperatorHandle const&, c10::DispatchKeySet, std::vector<c10::IValue, std::allocator<c10::IValue> >*) const (0x7f692603890a in /fsx/users/bahuang/repos/pytorch_fsx/aten/src/ATen/core/dispatch/Dispatcher.h:652)
frame #50: <unknown function> (0x7f69260387f9 in /fsx/users/bahuang/repos/pytorch_fsx/aten/src/ATen/core/dispatch/Dispatcher.h:388)
frame #51: <unknown function> (0x7f69261af0ef in /fsx/users/bahuang/repos/pytorch_fsx/aten/src/ATen/core/PythonFallbackKernel.cpp:96)
frame #52: <unknown function> (0x7f69261aff2b in /fsx/users/bahuang/repos/pytorch_fsx/aten/src/ATen/core/boxing/BoxedKernel_impl.h:25)
frame #53: c10::BoxedKernel::callBoxed(c10::OperatorHandle const&, c10::DispatchKeySet, std::vector<c10::IValue, std::allocator<c10::IValue> >*) const (0x7f6932aadced in /fsx/users/bahuang/repos/pytorch_fsx/aten/src/ATen/core/boxing/BoxedKernel_impl.h:41)
frame #54: c10::KernelFunction::callBoxed(c10::OperatorHandle const&, c10::DispatchKeySet, std::vector<c10::IValue, std::allocator<c10::IValue> >*) const (0x7f6932aadd26 in /fsx/users/bahuang/repos/pytorch_fsx/aten/src/ATen/core/boxing/KernelFunction_impl.h:43)
frame #55: c10::Dispatcher::callBoxed(c10::OperatorHandle const&, std::vector<c10::IValue, std::allocator<c10::IValue> >*) const (0x7f6925fd6ab2 in /fsx/users/bahuang/repos/pytorch_fsx/aten/src/ATen/core/dispatch/Dispatcher.h:628)
frame #56: <unknown function> (0x7f6925fd6690 in /fsx/users/bahuang/repos/pytorch_fsx/aten/src/ATen/core/dispatch/Dispatcher.h:376)
frame #57: <unknown function> (0x7f692bf5b525 in /fsx/users/bahuang/repos/pytorch_fsx/aten/src/ATen/core/dispatch/Dispatcher.h:380)
frame #58: <unknown function> (0x7f692bf59fac in /fsx/users/bahuang/repos/pytorch_fsx/torch/csrc/jit/runtime/register_c10_ops.cpp:15)
frame #59: <unknown function> (0x7f692bf5af41 in /usr/include/c++/7/bits/std_function.h:316)
frame #60: std::function<void (std::vector<c10::IValue, std::allocator<c10::IValue> >&)>::operator()(std::vector<c10::IValue, std::allocator<c10::IValue> >&) const (0x7f6932ab9a0f in /usr/include/c++/7/bits/std_function.h:706)
frame #61: <unknown function> (0x7f6932aad541 in /fsx/users/bahuang/repos/pytorch_fsx/aten/src/ATen/core/stack.h:41)
frame #62: <unknown function> (0x7f6932ab3102 in /fsx/users/bahuang/repos/pytorch_fsx/torch/csrc/jit/python/pybind_utils.h:1206 (discriminator 1))
frame #63: <unknown function> (0x7f6932ab3943 in /fsx/users/bahuang/repos/pytorch_fsx/torch/csrc/jit/python/pybind_utils.h:1272)
frame #64: <unknown function> (0x7f6932a46120 in /fsx/users/bahuang/repos/pytorch_fsx/torch/csrc/jit/python/init.cpp:1767)
frame #65: <unknown function> (0x7f6932a997be in /fsx/users/bahuang/repos/pytorch_fsx/third_party/pybind11/include/pybind11/cast.h:1441)
frame #66: <unknown function> (0x7f6932a8a985 in /fsx/users/bahuang/repos/pytorch_fsx/third_party/pybind11/include/pybind11/cast.h:1410)
frame #67: <unknown function> (0x7f6932a66e1e in /fsx/users/bahuang/repos/pytorch_fsx/third_party/pybind11/include/pybind11/pybind11.h:249)
frame #68: <unknown function> (0x7f6932a66ec2 in /fsx/users/bahuang/repos/pytorch_fsx/third_party/pybind11/include/pybind11/pybind11.h:224)
frame #69: <unknown function> (0x7f6932473111 in /fsx/users/bahuang/repos/pytorch_fsx/third_party/pybind11/include/pybind11/pybind11.h:929)
frame #104: __libc_start_main (0x7f693485dc87 in /build/glibc-uZu3wS/glibc-2.27/csu/../csu/libc-start.c:310)
```


[ghstack-poisoned]

c10/util/Backtrace.cpp

@@ -127,9 +127,7 @@ std::string rstrip(const std::string& s) {
 
 bool use_addr2line() {
   static bool _use_addr2line = []() {
-    return c10::utils::check_env("TORCH_SHOW_CPP_STACKTRACES_WITH_LINENO") ==
-        true &&
-        !system("which addr2line > /dev/null 2>&1");
+    return c10::utils::check_env("TORCH_SHOW_CPP_STACKTRACES_WITH_LINENO") == true;
   }();
   return _use_addr2line;
 }