cuda.core: VirtualMemoryResource.modify_allocation(config=...) changes the resource's policy

[Andy-Jost]

modify_allocation(buf, size, config=...) assigns the config to the resource before any check, so later allocate() calls and the accessibility properties use it. The docstring describes a per-call policy; test_vmm_allocator_policy_configuration asserts the persistence (see #1300). The assignment also skips the constructor's validation (RDMA support, host-location handling, dict configs). Decide per-call or persistent, then align the docstring and the test.