Clarify specification of SLICE operator on a NHWC or NWC tensor where N > 1

We get an error when command stream SLICE is invoked on a NHWC or NWC tensor where N > 1 (requiring either an explicit copy or the disabling a fusion optimisation in remove_splitsliceread in tflite_graph_optimiser.py)

If this is expected, then we should probably update the official documentation here https://review.mlplatform.org/plugins/gitiles/ml/ethos-u/ethos-u-vela/+/HEAD/SUPPORTED_OPS.md#tflite-generic-constraints , which includes SLICE in the set of operators excluded from the batch size = 1 requirement

IFM Tensor batch size must be 1 - [FULLY_CONNECTED, RESHAPE, SHAPE, SLICE, SOFTMAX, SPLIT, SPLIT_V, SQUEEZE, STRIDED_SLICE, UNPACK]