Remove preprocessing base classes we no longer use (#1824)

For some reason, all our causal lm models had a preprocessing
layer base class that packed inputs without setting things up for
the causal language model loss.

With our new base classes, `CausalLMPreprocessor` and
`Seq2SeqLMPreprocessor`, these are no longer used at all.

These are public, but I did a search for them on GitHub and the
only one I could find with any usage is `GPT2Preprocessor`. The
others are complete unused, and not generally very useful.

Let's remove them before anyone does depend on them so that we can
keep preprocessing one-to-one paired with a task. E.g.

```
BertTextClassifier
BertTextClassifierPreprocesssor
```

Author: mattdangerw

keras_nlp/api/models/__init__.py

@@ -34,7 +34,6 @@
 from keras_nlp.src.models.albert.albert_tokenizer import AlbertTokenizer
 from keras_nlp.src.models.backbone import Backbone
 from keras_nlp.src.models.bart.bart_backbone import BartBackbone
-from keras_nlp.src.models.bart.bart_preprocessor import BartPreprocessor
 from keras_nlp.src.models.bart.bart_seq_2_seq_lm import BartSeq2SeqLM
 from keras_nlp.src.models.bart.bart_seq_2_seq_lm_preprocessor import (
     BartSeq2SeqLMPreprocessor,
@@ -58,7 +57,6 @@
 from keras_nlp.src.models.bloom.bloom_causal_lm_preprocessor import (
     BloomCausalLMPreprocessor,
 )
-from keras_nlp.src.models.bloom.bloom_preprocessor import BloomPreprocessor
 from keras_nlp.src.models.bloom.bloom_tokenizer import BloomTokenizer
 from keras_nlp.src.models.causal_lm import CausalLM
 from keras_nlp.src.models.causal_lm_preprocessor import CausalLMPreprocessor
@@ -105,9 +103,6 @@
     DistilBertTokenizer,
 )
 from keras_nlp.src.models.electra.electra_backbone import ElectraBackbone
-from keras_nlp.src.models.electra.electra_preprocessor import (
-    ElectraPreprocessor,
-)
 from keras_nlp.src.models.electra.electra_tokenizer import ElectraTokenizer
 from keras_nlp.src.models.f_net.f_net_backbone import FNetBackbone
 from keras_nlp.src.models.f_net.f_net_masked_lm import FNetMaskedLM
@@ -127,14 +122,12 @@
 from keras_nlp.src.models.falcon.falcon_causal_lm_preprocessor import (
     FalconCausalLMPreprocessor,
 )
-from keras_nlp.src.models.falcon.falcon_preprocessor import FalconPreprocessor
 from keras_nlp.src.models.falcon.falcon_tokenizer import FalconTokenizer
 from keras_nlp.src.models.gemma.gemma_backbone import GemmaBackbone
 from keras_nlp.src.models.gemma.gemma_causal_lm import GemmaCausalLM
 from keras_nlp.src.models.gemma.gemma_causal_lm_preprocessor import (
     GemmaCausalLMPreprocessor,
 )
-from keras_nlp.src.models.gemma.gemma_preprocessor import GemmaPreprocessor
 from keras_nlp.src.models.gemma.gemma_tokenizer import GemmaTokenizer
 from keras_nlp.src.models.gpt2.gpt2_backbone import GPT2Backbone
 from keras_nlp.src.models.gpt2.gpt2_causal_lm import GPT2CausalLM
@@ -148,23 +141,18 @@
 from keras_nlp.src.models.gpt_neo_x.gpt_neo_x_causal_lm_preprocessor import (
     GPTNeoXCausalLMPreprocessor,
 )
-from keras_nlp.src.models.gpt_neo_x.gpt_neo_x_preprocessor import (
-    GPTNeoXPreprocessor,
-)
 from keras_nlp.src.models.gpt_neo_x.gpt_neo_x_tokenizer import GPTNeoXTokenizer
 from keras_nlp.src.models.llama3.llama3_backbone import Llama3Backbone
 from keras_nlp.src.models.llama3.llama3_causal_lm import Llama3CausalLM
 from keras_nlp.src.models.llama3.llama3_causal_lm_preprocessor import (
     Llama3CausalLMPreprocessor,
 )
-from keras_nlp.src.models.llama3.llama3_preprocessor import Llama3Preprocessor
 from keras_nlp.src.models.llama3.llama3_tokenizer import Llama3Tokenizer
 from keras_nlp.src.models.llama.llama_backbone import LlamaBackbone
 from keras_nlp.src.models.llama.llama_causal_lm import LlamaCausalLM
 from keras_nlp.src.models.llama.llama_causal_lm_preprocessor import (
     LlamaCausalLMPreprocessor,
 )
-from keras_nlp.src.models.llama.llama_preprocessor import LlamaPreprocessor
 from keras_nlp.src.models.llama.llama_tokenizer import LlamaTokenizer
 from keras_nlp.src.models.masked_lm import MaskedLM
 from keras_nlp.src.models.masked_lm_preprocessor import MaskedLMPreprocessor
@@ -173,16 +161,12 @@
 from keras_nlp.src.models.mistral.mistral_causal_lm_preprocessor import (
     MistralCausalLMPreprocessor,
 )
-from keras_nlp.src.models.mistral.mistral_preprocessor import (
-    MistralPreprocessor,
-)
 from keras_nlp.src.models.mistral.mistral_tokenizer import MistralTokenizer
 from keras_nlp.src.models.opt.opt_backbone import OPTBackbone
 from keras_nlp.src.models.opt.opt_causal_lm import OPTCausalLM
 from keras_nlp.src.models.opt.opt_causal_lm_preprocessor import (
     OPTCausalLMPreprocessor,
 )
-from keras_nlp.src.models.opt.opt_preprocessor import OPTPreprocessor
 from keras_nlp.src.models.opt.opt_tokenizer import OPTTokenizer
 from keras_nlp.src.models.pali_gemma.pali_gemma_backbone import (
     PaliGemmaBackbone,
@@ -201,7 +185,6 @@
 from keras_nlp.src.models.phi3.phi3_causal_lm_preprocessor import (
     Phi3CausalLMPreprocessor,
 )
-from keras_nlp.src.models.phi3.phi3_preprocessor import Phi3Preprocessor
 from keras_nlp.src.models.phi3.phi3_tokenizer import Phi3Tokenizer
 from keras_nlp.src.models.preprocessor import Preprocessor
 from keras_nlp.src.models.roberta.roberta_backbone import RobertaBackbone