[Exec] Add a script to test GPU memory bandwidth

[junrushao]

This PR adds a script to test GPU memory bandwidth in TVM. Example usage:

python -m tvm.exec.gpu_memory_bandwidth \
    "nvidia/geforce-rtx-3090-ti"        \
    --dtype "float32"                   \
    --bx "8,16,32,64,128,256"           \
    --tx "32,64,128,256,512,1024"       \
    --vec "1,2,4"

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