[DistGB] remove utils.toindex() from DistDGL

[Rhett-Ying]

🔨Work Item

IMPORTANT:

• This template is only for dev team to track project progress. For feature request or bug report, please use the corresponding issue templates.
• DO NOT create a new work item if the purpose is to fix an existing issue or feature request. We will directly use the issue in the project tracker.

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

Description

utils.toindex() always use int64. For most of the use case, we could replace with native pytorch tensor or numpy array. This is required by int32/64 dtype of DistGraph, especially for GraphBolt.

Depending work items or issues