Enable reshape of _ScopedAllocatorConcat output.

The _ScopedAllocatorConcat kernel outputs the backing tensor after performing
runtime bounds checks.  However, the shape of the backing tensor may not match
the desired output shape of the concat operation.

This change adds a "reshape" boolean attribute to _ScopedAllocatorConcat kernel.
When this attribute is set to true, the kernel outputs a reshaped backing tensor
according to the "shape" attribute.

PiperOrigin-RevId: 195169105