Update Gather-7 specification

[achetver]

Details:

• Update Gather-7 operation specification since we decide to support negative values of batch_dims attribute. Also update formula to same view as in TF specification to make it more clear.

Tickets:

• 54329

[achetver]

@sadolini @yekruglov @iimironov @lazarevevgeny Please, review this PR with important changes in Gather-7 operation specification.

[a-sidorova]

I think that description for axis is incorrect:
Allowed values are from [-len(data.shape), len(indices.shape) - 1] and axis >= batch_dims

My suggestion is:
Allowed values are from [-len(data.shape), len(data.shape) - 1] and axis >= batch_dims

Because otherwise the example 4 contradicts this description. axis = 2 but len(indices.shape) - 1 = 2 - 1 =1.

[andrei-cv]

Allowed values are from [-len(indices.shape), len(indices.shape) - 1] and axis >= batch_dims as in tensorflow

[pavel-esir]

Allowed values are from [-len(indices.shape), len(indices.shape) - 1] and axis >= batch_dims as in tensorflow

@andrei-cv indeed there must be data.shape not indices. Axis value is not bound to indices. In TF for axis allowed values are
Allowed values are from [-len(data.shape), len(data.shape) - 1] and axis >= batch_dims

https://github.com/tensorflow/tensorflow/blob/master/tensorflow/core/kernels/gather_op.cc#L81-L89
there params goes for 1st input in our case means data