[feat] add default download root instead of using current working dir

[geniuspatrick]

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Motivation

Set the default download root path as ~/.mindspore(hereinafter referred to as ${DOWNLOAD_ROOT}) instead of using the current working directory.
For different types of files, we place them in:

• Checkpoints of pretrained model: ${DOWNLOAD_ROOT}/models
• Small datasets like mnist cifair10: ${DOWNLOAD_ROOT}/datasets

We can change the default download path with function set_default_download_root, or explicitly pass in the download path when calling function download_and_extract_archive or download_url.

Test Plan

Already in tests.

Related Issues and PRs

None