feat(python): Clarify interaction between the CDeviceArray, the CArra…

…yView, and the CArray (#409)

When device support was first added, the `CArrayView` was device-aware
but the `CArray` was not. This worked well until it was clear that
`__arrow_c_array__` needed to error if it did not represent a CPU array
(and the `CArray` had no way to check). Now, the `CArray` has a
`device_type` and `device_id`. A nice side-effect of this is that we get
back the `view()` method (whose removal @jorisvandenbossche had
lamented!).

This also implements the device array protocol to help test
apache/arrow#40717 . This protocol isn't
finalized yet and I could remove that part until it is (although it
doesn't seem likely to change).

The non-cpu case is still hard to test without real-world CUDA
support...this PR is just trying to get the right information in the
right place as early as possible.

```python
import nanoarrow as na

array = na.c_array([1, 2, 3], na.int32())
array.device_type, array.device_id
#> (1, 0)
```

---------

Co-authored-by: Dane Pitkin <[email protected]>

extensions/nanoarrow_device/src/nanoarrow/nanoarrow_device.c

@@ -115,7 +115,7 @@ struct ArrowDevice* ArrowDeviceCpu(void) {
 
 void ArrowDeviceInitCpu(struct ArrowDevice* device) {
   device->device_type = ARROW_DEVICE_CPU;
-  device->device_id = 0;
+  device->device_id = -1;
   device->array_init = NULL;
   device->array_move = NULL;
   device->buffer_init = &ArrowDeviceCpuBufferInit;
@@ -135,7 +135,7 @@ struct ArrowDevice* ArrowDeviceCuda(ArrowDeviceType device_type, int64_t device_
 #endif
 
 struct ArrowDevice* ArrowDeviceResolve(ArrowDeviceType device_type, int64_t device_id) {
-  if (device_type == ARROW_DEVICE_CPU && device_id == 0) {
+  if (device_type == ARROW_DEVICE_CPU) {
     return ArrowDeviceCpu();
   }
 

extensions/nanoarrow_device/src/nanoarrow/nanoarrow_device_test.cc

@@ -28,7 +28,7 @@ TEST(NanoarrowDevice, CheckRuntime) {
 TEST(NanoarrowDevice, CpuDevice) {
   struct ArrowDevice* cpu = ArrowDeviceCpu();
   EXPECT_EQ(cpu->device_type, ARROW_DEVICE_CPU);
-  EXPECT_EQ(cpu->device_id, 0);
+  EXPECT_EQ(cpu->device_id, -1);
   EXPECT_EQ(cpu, ArrowDeviceCpu());
 
   void* sync_event = nullptr;