Bug in `_grow_allocation_fast_path`

[mdboom]

I'm ramping up .pyi generation, which lead to type checking our code for the first time, and it's discovered a potential bug at this line:

https://github.com/NVIDIA/cuda-python/blob/main/cuda_core/cuda/core/_memory/_virtual_memory_resource.py#L338

cuda_core/cuda/core/_memory/_virtual_memory_resource.py:338: error: "Buffer" has no attribute "_size"; maybe "size"?

Indeed, Buffer does not expose _size to Python (and _virtual_memory_resource is Python, not Cython).

(Pdb) p buffer._size
*** AttributeError: 'cuda.core._memory._buffer.Buffer' object has no attribute '_size'. Did you mean: 'size'?
(Pdb) buffer._size = 42
*** AttributeError: 'cuda.core._memory._buffer.Buffer' object has no attribute '_size' and no __dict__ for setting new attributes. Did you mean: 'size'?

It doesn't look like we have any test coverage for this file.

We probably need to make Buffer.size a read/write property.

[leofang]

Interesting, this path (_grow_allocation_fast_path) should be covered by this test

cuda-python/cuda_core/tests/test_memory.py

Line 950 in 2ddbe6b

grown_buffer = vmm_mr.modify_allocation(buffer, 4 * 1024 * 1024)

not sure what's missing.