[ssl] add layerdropout for fintune

[Mddct]

Not for merge for now (too many auto-format issues, just a example)
see:

wenet/wenet/transformer/encoder.py

Lines 456 to 472 in cd54ccc

	if layer_dropout_rate == 0.0:
	self.encoders = torch.nn.ModuleList([])
	else:
	self.encoders = LayerDropModuleList(p=dropout_rate)
	self.encoders.extend([
	ConformerEncoderLayer(
	output_size,
	encoder_selfattn_layer(*encoder_selfattn_layer_args),
	positionwise_layer(*positionwise_layer_args),
	positionwise_layer(
	*positionwise_layer_args) if macaron_style else None,
	convolution_layer(
	*convolution_layer_args) if use_cnn_module else None,
	dropout_rate,
	normalize_before,
	) for _ in range(num_blocks)
	])

ref:

• wav2vec: https://github.com/facebookresearch/fairseq/blob/da8fb630880d529ab47e53381c30ddc8ad235216/examples/wav2vec/config/finetuning/vox_1h_3.yaml#L72
• hubert: https://github.com/facebookresearch/fairseq/blob/da8fb630880d529ab47e53381c30ddc8ad235216/examples/hubert/config/finetune/base_10h.yaml#L74