Standard weekly patch release

[1.4.9] - 2021-09-30

• Moved the gradient unscaling in NativeMixedPrecisionPlugin from pre_optimizer_step to post_backward (#9606)
• Fixed gradient unscaling being called too late, causing gradient clipping and gradient norm tracking to be applied incorrectly (#9606)
• Fixed lr_find to generate same results on multiple calls (#9704)
• Fixed reset metrics on validation epoch end (#9717)
• Fixed input validation for gradient_clip_val, gradient_clip_algorithm, track_grad_norm and terminate_on_nan Trainer arguments (#9595)
• Reset metrics before each task starts (#9410)

Contributors

@rohitgr7 @tchaton

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