taichi: [CUDA] detected to be supported and crash on card without unified memory

Describe the bug CUDA detected to be SUPPORTED on a machine without CUDA. It’s because is_cuda_api_avaliable returned true even if I don’t have CUDA.

Log/Screenshots

(yuanming-hu/glfw) [bate@archit taichi]$ python examples/mpm128.py  
[Taichi] mode=development
[Taichi] preparing sandbox at /tmp/taichi-7haz507t
[Taichi] sandbox prepared
[Taichi] <dev mode>, supported archs: [cpu, cuda, opengl], commit 4e2e5605, python 3.8.2
[Hint] Use WSAD/arrow keys to control gravity. Use left/right mouse bottons to attract/repel. Press R to reset.
[W 04/13/20 09:29:21.266] [cuda_driver.h:call_with_warning@60] CUDA Error CUDA_ERROR_INVALID_DEVICE: invalid device ordinal while calling mem_advise (cuMemAdvise)
[E 04/13/20 09:29:21.860] Received signal 7 (Bus error)


***********************************
* Taichi Compiler Stack Traceback *                                                          
***********************************                                                          
/tmp/taichi-7haz507t/taichi_core.so: taichi::Logger::error(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, bool)                                  
/tmp/taichi-7haz507t/taichi_core.so: taichi::signal_handler(int)                             
/usr/lib/libc.so.6(+0x3bd70) [0x7f359062bd70]                                                
/tmp/taichi-7haz507t/taichi_core.so: taichi::lang::MemoryPool::daemon()
/usr/lib/libstdc++.so.6(+0xcfb24) [0x7f357ff41b24]
/usr/lib/libpthread.so.0(+0x946f) [0x7f359021746f]
/usr/lib/libc.so.6: clone
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Attaching to process 8383
[New LWP 8388]
[New LWP 8389]
[New LWP 8390]
[New LWP 8391]
[New LWP 8396]
[New LWP 8397]
[New LWP 8398]
[New LWP 8399]
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/usr/lib/libthread_db.so.1".
0x00007f3580fc910c in llvm::Twine::toVector(llvm::SmallVectorImpl<char>&) const ()
   from /tmp/taichi-7haz507t/taichi_core.so
(gdb) 

To Reproduce Just run the example/mpm128.py.

If you have local commits (e.g. compile fixes before you reproduce the bug), please make sure you first make a PR to fix the build errors and then report the bug.