better singleton already made warnings

[piotrm0]

Other details that are good to know but need not be announced:

• Added a better warning message when creating an instance of a singleton class that was already made. For example:

from trulens_eval import OpenAI
from trulens_eval.feedback import Groundedness

g = Groundedness()
OpenAI(client=..., something=42)

Will print out

  OpenAIClient singleton already made, ignoring arguments {'something': 42}
  Singleton instance of type OpenAIEndpoint already created at:
  /var/folders/2j/xg0_cv993gl37s_8qs5qrdxm0000gn/T/ipykernel_84662/360095262.py:8
	  OpenAI(client=..., something=42)
  
  You can delete the singleton by calling `<instance>.delete_singleton()` or 
    ```python
    from trulens_eval.utils.python import SingletonPerName
    SingletonPerName.delete_singleton_by_name(name="openai", cls=OpenAIEndpoint)
    ```

The first part is the existing warning but the second points out the source code line that caused the singleton to be created to begin with. In this case, the call to Groundedness created a default openai client hence the second one's configuration is ignored. Note the weird source code file name is because this is in a jupyter notebook running on vscode.

Added this warning to OpenAIEndpoint, LiteLLMEndpoint, and Tru.

• The instructions also show how to delete the instance, thus this code is ok:

g = Groundedness()
g.groundedness_provider.endpoint.delete_singleton()
OpenAI(client=..., something=42)

• This is also ok:

g = Groundedness()
from trulens_eval.utils.python import SingletonPerName
from trulens_eval.feedback.provider.endpoint.openai import OpenAIEndpoint
SingletonPerName.delete_singleton_by_name(name="openai", cls=OpenAIEndpoint)
OpenAI(client=..., something=42)