[CANN] Simplify the environment variable setting for GGML_CANN_MEM_POOL and GGML_CANN_ASYNC_MODE

[bachelor-dou]

This PR simplifies the environment variable setup for GGML_CANN_MEM_POOL and GGML_CANN_ASYNC_MODE.

GGML_CANN_MEM_POOL:

• default VMM
• By setting export GGML_CANN_MEM_POOL="pRio" (the value is case-insensitive), you specify the use of a priority queue-based memory pool.
• export GGML_CANN_MEM_POOL="legacy" or any other value will enable the legacy pool.

GGML_CANN_ASYNC_MODE:

• Yes, enable, y, 1, on (case insensitive) are all valid values to enable GGML_CANN_ASYNC_MODE, such as export GGML_CANN_ASYNC_MODE=yEs.

[bachelor-dou]

@hipudding

[hipudding]

Update doc.

[hipudding]

GGML_CANN_ASYNC_MODE

Enables asynchronous operator submission. Disabled by default.

GGML_CANN_MEM_POOL

Specifies the memory pool management strategy:

• vmm: Utilizes a virtual memory manager pool. If hardware support for VMM is unavailable, falls back to the legacy (leg) memory pool.
• prio: Employs a priority queue-based memory pool management.
• leg: Uses a fixed-size buffer pool.

GGML_CANN_DISABLE_BUF_POOL_CLEAN

Controls automatic cleanup of the memory pool. This option is only effective when using the prio or leg memory pool strategies.