[v2.0] RNN: use rnn_params

[barry-jin]

Description

Use only one parameter for rnn_layer in stead of multiple parameters to improve performance.

Checklist

Essentials

• PR's title starts with a category (e.g. [BUGFIX], [MODEL], [TUTORIAL], [FEATURE], [DOC], etc)
• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage
• Code is well-documented

Changes

• Fused rnn parameter to only one parameter
• Add rnn parameter split function in gluon.utils to help users split rnn_param into weight and bias in different layers.

Comments

• If this change is a backward incompatible change, why must this change be made.
• Interesting edge cases to note here

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[szha]

we will need an initializer for the fused parameter and use it as default. With this default initializer for RNN layers, the bias terms should be initialized as 0s.

[barry-jin]

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[mxnet-bot]

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[barry-jin]

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[barry-jin]

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[mxnet-bot]

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