[refactor] Turn off stack traceback info by default

[ailzhang]

When Taichi program crashes we used to show a Taichi Compiler Stack Traceback (a.k.a purple stack, example shown below) but this stack doesn't have enough symbol for developers, nor
it's readable for normal Taichi python users. So let's turn it off by
default to avoid confusion.

***********************************
* Taichi Compiler Stack Traceback *
***********************************
/home/ailing/github/taichi/python/taichi/_lib/core/taichi_core.cpython-38-x86_64-linux-gnu.so: taichi::Logger::error(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, bool)
/home/ailing/github/taichi/python/taichi/_lib/core/taichi_core.cpython-38-x86_64-linux-gnu.so(+0x503753) [0x7f0d4bd19753]
/home/ailing/github/taichi/python/taichi/_lib/core/taichi_core.cpython-38-x86_64-linux-gnu.so(+0x503317) [0x7f0d4bd19317]
/home/ailing/github/taichi/python/taichi/_lib/core/taichi_core.cpython-38-x86_64-linux-gnu.so(+0x808a2d) [0x7f0d4c01ea2d]
/home/ailing/github/taichi/python/taichi/_lib/core/taichi_core.cpython-38-x86_64-linux-gnu.so(+0x7194b5) [0x7f0d4bf2f4b5]
python(+0x13c8ee) [0x55db2635b8ee]
python(_PyObject_MakeTpCall+0x3bf) [0x55db2635075f]
python(+0x166a80) [0x55db26385a80]
python(_PyEval_EvalFrameDefault+0x4fb3) [0x55db263f9b63]
python(_PyEval_EvalCodeWithName+0x260) [0x55db263eb480]
python(_PyFunction_Vectorcall+0x594) [0x55db263eca44]
python(_PyEval_EvalFrameDefault+0x4c0) [0x55db263f5070]
python(_PyEval_EvalCodeWithName+0xd52) [0x55db263ebf72]
python(_PyFunction_Vectorcall+0x594) [0x55db263eca44]
python(_PyEval_EvalFrameDefault+0x71b) [0x55db263f52cb]
python(_PyEval_EvalCodeWithName+0x260) [0x55db263eb480]
python(PyEval_EvalCode+0x23) [0x55db263ecd33]
python(+0x2414a2) [0x55db264604a2]
python(+0x252292) [0x55db26471292]
python(+0x25542b) [0x55db2647442b]
python(PyRun_SimpleFileExFlags+0x1bf) [0x55db2647460f]
python(Py_RunMain+0x3a9) [0x55db26474ae9]
python(Py_BytesMain+0x39) [0x55db26474ce9]
/lib/x86_64-linux-gnu/libc.so.6: __libc_start_main
python(+0x1f7847) [0x55db26416847]

Just in case someone still needs it, it can be brought back if you do
ti.init(debug=True).

Related issue = #

[netlify]

✅ Deploy Preview for docsite-preview ready!

Name	Link
🔨 Latest commit	9baa72e
🔍 Latest deploy log	https://app.netlify.com/sites/docsite-preview/deploys/62c5422348a13d0009bff647
😎 Deploy Preview	https://deploy-preview-5347--docsite-preview.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site settings.

[lin-hitonami]

LGTM!