Dedicated ML hardware accelerators: NPU, VPU, xPU

[anssiko]

(Related to the WebML WG Charter in development at #19)

The initial version of WebNN specifies two device types, "cpu" and "gpu".

However, the API is extensible with new device types and in our discussion support for NPU, VPU, or XPU has come up as a new "v2" feature.

The initial charter refers to "dedicated ML hardware accelerators" in its Motivation and Background, but if this is important we could be more explicit regarding NPU/VPU/XPU device type support.

@wchao1115 @huningxin for feedback.

[anssiko]

I'm hearing the WG considers the current charter text inclusive enough to allow the WebNN API to accommodate new device types in its future versions. No charter scope changes suggested for this issue.