[MPS] Initialize MPSDevice::_mtl_device property to nil (pytorch#…

…78136)

This prevents `import torch` accidentally crash on machines with no metal devices

Should prevent crashes reported in pytorch#77662 (comment) and https://github.com/pytorch/functorch/runs/6560056366?check_suite_focus=true

Backtrace to the crash:
```
(lldb) bt
* thread #1, stop reason = signal SIGSTOP
  * frame #0: 0x00007fff7202be57 libobjc.A.dylib`objc_msgSend + 23
    frame #1: 0x000000010fd9f524 libtorch_cpu.dylib`at::mps::HeapAllocator::MPSHeapAllocatorImpl::MPSHeapAllocatorImpl() + 436
    frame #2: 0x000000010fda011d libtorch_cpu.dylib`_GLOBAL__sub_I_MPSAllocator.mm + 125
    frame #3: 0x000000010ada81e3 dyld`ImageLoaderMachO::doModInitFunctions(ImageLoader::LinkContext const&) + 535
    frame #4: 0x000000010ada85ee dyld`ImageLoaderMachO::doInitialization(ImageLoader::LinkContext const&) + 40(lldb) up
frame #1: 0x000000010fd9f524 libtorch_cpu.dylib`at::mps::HeapAllocator::MPSHeapAllocatorImpl::MPSHeapAllocatorImpl() + 436
libtorch_cpu.dylib`at::mps::HeapAllocator::MPSHeapAllocatorImpl::MPSHeapAllocatorImpl:
->  0x10fd9f524 <+436>: movq   %rax, 0x1b0(%rbx)
    0x10fd9f52b <+443>: movw   $0x0, 0x1b8(%rbx)
    0x10fd9f534 <+452>: addq   $0x8, %rsp
    0x10fd9f538 <+456>: popq   %rbx
(lldb) disassemble
 ...
    0x10fd9f514 <+420>: movq   0xf19ad15(%rip), %rsi     ; "maxBufferLength"
    0x10fd9f51b <+427>: movq   %r14, %rdi
    0x10fd9f51e <+430>: callq  *0xeaa326c(%rip)          ; (void *)0x00007fff7202be40: objc_msgSend
```

which corresponds to `[m_device maxBufferLength]` call, where `m_device` is not initialized in
https://github.com/pytorch/pytorch/blob/2ae3c59e4bcb8e6e75b4a942cacc2d338c88e609/aten/src/ATen/mps/MPSAllocator.h#L171

Pull Request resolved: pytorch#78136
Approved by: https://github.com/seemethere

aten/src/ATen/mps/MPSDevice.mm

@@ -20,7 +20,7 @@
   _mtl_device = nil;
 }
 
-MPSDevice::MPSDevice() {
+MPSDevice::MPSDevice(): _mtl_device(nil) {
   NSArray* devices = [MTLCopyAllDevices() autorelease];
   for (unsigned long i = 0 ; i < [devices count] ; i++) {
     id<MTLDevice>  device = devices[i];