[lang] Hide dtype and needs_grad from SNode

[strongoier]

Related issue = #3782

Both of them only apply to leaf SNodes, i.e., fields. So users should get the info from a Field instance instead of a SNode. After this PR, dtype of SNode is hidden and needs_grad is removed.

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[ailzhang]

LGTM!

[ifsheldon]

@ailzhang @strongoier I need needs_grad in my library stannum. This property dates back to #2498 and I had the conversation in the old slack channel of Taichi community. Removing this break my library silently....... Can you add it back and add a note in its inline doc that it was added in #2498 for a reason. And I cannot now write field.snode.ptr.has_grad() as a workaround.