Make BaseStringMessagePromptTemplate.from_template return type generic (#4523)

# Make BaseStringMessagePromptTemplate.from_template return type generic

I use mypy to check type on my code that uses langchain. Currently after
I load a prompt and convert it to a system prompt I have to explicitly
cast it which is quite ugly (and not necessary):
```
prompt_template = load_prompt("prompt.yaml")
system_prompt_template = cast(
    SystemMessagePromptTemplate,
    SystemMessagePromptTemplate.from_template(prompt_template.template),
)
```

With this PR, the code would simply be: 
```
prompt_template = load_prompt("prompt.yaml")
system_prompt_template = SystemMessagePromptTemplate.from_template(prompt_template.template)
```

Given how much langchain uses inheritance, I think this type hinting
could be applied in a bunch more places, e.g. load_prompt also return a
`FewShotPromptTemplate` or a `PromptTemplate` but without typing the
type checkers aren't able to infer that. Let me know if you agree and I
can take a look at implementing that as well.

        @hwchase17 - project lead

        DataLoaders
        - @eyurtsev

langchain/prompts/chat.py

@@ -3,7 +3,7 @@ from __future__ import annotations
 
 from abc import ABC, abstractmethod
 from pathlib import Path
-from typing import Any, Callable, List, Sequence, Tuple, Type, Union
+from typing import Any, Callable, List, Sequence, Tuple, Type, TypeVar, Union
 
 from pydantic import BaseModel, Field
 
@@ -58,12 +58,19 @@ class MessagesPlaceholder(BaseMessagePromptTemplate):
         return [self.variable_name]
 
 
+MessagePromptTemplateT = TypeVar(
+    "MessagePromptTemplateT", bound="BaseStringMessagePromptTemplate"
+)
+
+
 class BaseStringMessagePromptTemplate(BaseMessagePromptTemplate, ABC):
     prompt: StringPromptTemplate
     additional_kwargs: dict = Field(default_factory=dict)
 
     @classmethod
-    def from_template(cls, template: str, **kwargs: Any) -> BaseMessagePromptTemplate:
+    def from_template(
+        cls: Type[MessagePromptTemplateT], template: str, **kwargs: Any
+    ) -> MessagePromptTemplateT:
         prompt = PromptTemplate.from_template(template)
         return cls(prompt=prompt, **kwargs)