fix: Target docs (#704)

Replace ``_target`` with ``_target_``.
Fix YAML indentation.

## Description
<!-- What issue or task does this change relate to? -->
Typo(s) in the docs.

## What problem does this change solve?
<!-- Describe if it's a bugfix, new feature, doc update, or breaking
change -->
Bugfix

## What issue or task does this change relate to?
<!-- link to Issue Number -->
N/A

##  Additional notes ##
<!-- Include any additional information, caveats, or considerations that
the reviewer should be aware of. -->
I think that this reference to `_target` is a typo, which should say
`_target_`.
I believe that some YAML blocks are not valid (as tested with
`yaml.safe_load(the_text)`) and that YAML objects can either be lists or
dicts but not a mixture of both.
Not valid (raises ParserError):

```yaml
a_key:
  - a list item
  another_key: a value
```

Valid:

```yaml
a_key:
- a_list_of_dicts_key_1: val
  a_list_of_dicts_key_2: val
```

[rstcheck](https://github.com/rstcheck/rstcheck/tree/main) catches these
(and a few others). I'd be happy to add it to the CI system if that's
something you'd be interested in.

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---------

Co-authored-by: Matthew Chantry <[email protected]>

Author: Iain-S

graphs/docs/graphs/post_processor.rst

@@ -42,8 +42,8 @@ attribute where the masking indices will be stored.
 
    post_processors:
    - _target_: anemoi.graphs.processors.RemoveUnconnectedNodes
-      nodes_name: data
-      save_mask_indices_to_attr: indices_connected_nodes # optional
+     nodes_name: data
+     save_mask_indices_to_attr: indices_connected_nodes # optional
 
 The ``RemoveUnconnectedNodes`` post-processor also supports an
 ``ignore`` argument, which is optional but highly convenient in certain
@@ -62,9 +62,9 @@ preserved. For example:
 
    post_processors:
    - _target_: anemoi.graphs.processors.RemoveUnconnectedNodes
-      nodes_name: data
-      ignore: important_nodes
-      save_mask_indices_to_attr: indices_connected_nodes # optional
+     nodes_name: data
+     ignore: important_nodes
+     save_mask_indices_to_attr: indices_connected_nodes # optional
 
 In this configuration, any node with the attribute `important_nodes` set
 will not be pruned, regardless of its connectivity status.
@@ -87,8 +87,8 @@ retained in the graph.
 
    post_processors:
    - _target_: anemoi.graphs.processors.SubsetNodesInArea
-      nodes_name: [data, hidden]
-      area: [40, 10, 20, 30] # (north, west, south, east)
+     nodes_name: [data, hidden]
+     area: [40, 10, 20, 30] # (north, west, south, east)
 
 The area is defined by four values representing the northern, western,
 southern, and eastern boundaries, in that order: `(north, west, south,
@@ -171,7 +171,7 @@ can be configured to sort in either ascending or descending order:
 
    post_processors:
    - _target_: anemoi.graphs.processors.SortEdgeIndexBySourceNodes
-      descending: True  # optional, defaults to true
+     descending: True  # optional, defaults to true
 
 SortEdgeIndexByTargetNodes
 ==========================
@@ -183,7 +183,7 @@ indices based on the target nodes:
 
    post_processors:
    - _target_: anemoi.graphs.processors.SortEdgeIndexByTargetNodes
-      descending: True  # optional, defaults to true
+     descending: True  # optional, defaults to true
 
 Both processors maintain the consistency of all edge attributes while
 sorting, ensuring that the relationship between edge indices and their

training/docs/modules/diagnostics.rst

@@ -30,7 +30,7 @@ Callbacks are configured in the config file under the
 
 For regular callbacks, they can be provided as a list of dictionaries
 underneath the ``config.diagnostics.callbacks`` key. Each dictionary
-must have a ``_target`` key which is used by hydra to instantiate the
+must have a ``_target_`` key which is used by hydra to instantiate the
 callback, any other kwarg is passed to the callback's constructor.
 
 .. code:: yaml