Adding GlobalAveragePooling1D

[Shashi456]

So I'm relatively new to the swift ecosystem, let me know if there are any errors. Afaik my understanding of GlobalAveragePooling in a single dimension is returning the mean * the no of batches if it's a multi-batch input.

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[rxwei]

Did you sign the CLA?

[Shashi456]

@rxwei yeah i have

[rxwei]

Suggested change

	/// Creates a global average pooling layer in 1 dimension
	/// Creates a global average pooling layer.

• Please end sentences with a period.
• Remove "in 1 dimension" to be consistent with the doc comment on the type declaration.