[GraphBolt][Hetero] Utilize single tensor with offsets instead of dict

## 🔨Work Item

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Project tracker: https://github.com/orgs/dmlc/projects/2

## Description

If we can utilize a single tensor along with offsets to store seeds or sampled edges, that can make our code more performant by avoiding loops over etypes on the Python side.

So that we can almost remove these two functions:

https://github.com/dmlc/dgl/blob/2f585940a80efd39639388dfc206498b3279e58d/python/dgl/graphbolt/impl/fused_csc_sampling_graph.py#L454-L469

We can use `gb.expand_indptr` to perform batched computations on the whole tensor in a single call to first broadcast the elements of the operation and then perform the computation.

@frozenbugs @peizhou001