Block interface: rework LM config, fine-grained initialization, lr_scale, peft

[jlamypoirier]

✨ Description

Rework LM config:

• Extract embedding and output layer configs.
• Rename tie_word_embeddings -> output_layer.tied_weight
• Position embeddings are now enabled through embeddings_layer.position_embeddings.enabled, always disabled by default independently of rotary embeddings.
• Rename max_position_embeddings -> embeddings_layer.num_position_embeddings
• Rename parallel_embeddings -> embeddings_layer.vocab_parallel

Rework initialization config:

• Remove most ad-hoc initialization arguments (leftovers from Block interface: extract mixer and mlp config #359)
• Add dynamic initialization config scheme so initialization may be arbitrarily configured.
• Add optional initialization config to all parameters. If not set, the default set by the parent layer will be used, matching previous behaviour.
• Mamba: remove dt_init, dt_scale as the same can be obtained through the new init config scheme. Replace dt_min, dt_max, dt_init_floor by the mamba_dt_bias initialization type with similar options.

Rework LR scales:

• Add lr_scale option to all parameters and most layers.
• LR scales combine multiplicatively, i.e. the actual LR scale for a given parameter is the multiplication of its lr scale and that of all its parent

Rework Peft (lora):

• Add apply_peft option to linear layers. If true, peft will be enabled for that layer (ex. wrapped with lora), otherwise the layer will be treated as non-peft (ex. frozen or ignored). If let unset, the default set by the parent layer will be used instead . (False except for attn query and value.)
• Remove transformer peft config, use peft config directly instead. (Was there to determine the peft layers, now handled in linear config)

Todo (next prs):

• (from Block interface: extract mixer and mlp config #359) Add back fine-grained bias enabling config (quen2 and dream disabled).
• (from Block interface: extract mixer and mlp config #359) Rework SSM conversion (disabled).
• (from Block interface: parameter and linear config, separate SSM config. #358) Allow separate configuration for concatenated layers (ex. key_value, ssm in_proj)
• LM config could use polishing

[tscholak]

LGTM!