Add empty ITensorNetwork / TreeTensorNetwork constructors

Mirror the `Vector()` / `Dictionary()` convention by providing
parameterless and `{V}`-only constructors that yield an empty network:

- `ITensorNetwork()` / `ITensorNetwork{V}()`
- `TreeTensorNetwork()` / `TreeTensorNetwork{V}()`

The default vertex type is `Any`. The body of `ITensorNetwork{V}(tensors)`
now seeds from `ITensorNetwork{V}()` and writes each tensor through
`setindex!`, removing the explicit field call and the prior
`set_vertex_data!` call at this layer.

Also drop the redundant `ITensorNetwork(::TreeTensorNetwork)` and the
`ITensorNetwork(::AbstractTTN)` "not implemented" placeholder — the
generic `ITensorNetwork(tensors)` now handles a `TreeTensorNetwork`
via `keytype` and `setindex!`. Update the docs to drop the dangling
references.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: mtfishman

docs/src/experimental_methods.md

@@ -37,9 +37,8 @@ Methods which still need to be discussed, modified, or deprecated.
 
 #### AbstractTreeTensorNetwork Type
 
-* Required-to-implement abstract interface — `TreeTensorNetwork` provides all three (`treetensornetworks/abstracttreetensornetwork.jl`):
+* Required-to-implement abstract interface — `TreeTensorNetwork` provides both (`treetensornetworks/abstracttreetensornetwork.jl`):
   ```julia
-  ITensorNetwork(tn::AbstractTTN)
   ortho_region(tn::AbstractTTN)
   set_ortho_region(tn::AbstractTTN, new_region)
   ```
@@ -134,11 +133,6 @@ Methods which still need to be discussed, modified, or deprecated.
 
 #### TreeTensorNetwork Type
 
-* Get the underlying `ITensorNetwork` of a `TTN` (drops orthogonality metadata) (`treetensornetworks/treetensornetwork.jl`):
-  ```julia
-  ITensorNetwork(tn::TTN)
-  ```
-
 * Get the current orthogonality region — the set of vertices forming the gauge center (`treetensornetworks/treetensornetwork.jl`):
   ```julia
   ortho_region(tn::TTN)

docs/src/tree_tensor_networks.md

@@ -65,7 +65,6 @@ itn_again = ITensorNetwork(psi)  # TTN → ITensorNetwork
 
 ```@docs; canonical=false
 ITensorNetworks.TreeTensorNetwork
-ITensorNetworks.ITensorNetwork(::ITensorNetworks.TreeTensorNetwork)
 ```
 
 ## Orthogonal Gauge

src/itensornetwork.jl

@@ -69,17 +69,24 @@ end
 # Constructors
 #
 
-# Construct by feeding `tensors` through `set_vertex_data!` one vertex
-# at a time — this centralizes the reverse-map registration, edge
-# inference, and hypergraph check in a single place (the `setindex!`
-# code path). Walking `keys(tensors)` in order makes the resulting
-# `neighbors(g, v)` / `edges(g)` iteration order deterministic in the
-# input order. An empty `tensors` (`Dict{V, ITensor}()`, etc.) yields
-# an empty network — there is no separate empty-arg constructor.
+# Empty network with no vertices. Mirrors the `Vector()` / `Dictionary()`
+# convention; vertex type defaults to `Any` when unspecified.
+function ITensorNetwork{V}() where {V}
+    return ITensorNetwork{V}(
+        NamedGraph{V}(), Dictionary{V, ITensor}(), Dict{Index, Set{V}}()
+    )
+end
+ITensorNetwork() = ITensorNetwork{Any}()
+
+# Construct by writing each tensor into a freshly empty network via
+# `setindex!`. The `setindex!` code path centralizes reverse-map
+# registration, edge inference, and the hypergraph check, and walking
+# `keys(tensors)` in order makes the resulting `neighbors(g, v)` /
+# `edges(g)` iteration order deterministic in the input order.
 function ITensorNetwork{V}(tensors) where {V}
-    tn = ITensorNetwork{V}(NamedGraph{V}(), Dictionary{V, ITensor}(), Dict{Index, Set{V}}())
+    tn = ITensorNetwork{V}()
     for v in keys(tensors)
-        set_vertex_data!(tn, tensors[v], v)
+        tn[v] = tensors[v]
     end
     return tn
 end

src/treetensornetworks/abstracttreetensornetwork.jl

@@ -16,8 +16,6 @@ function DataGraphs.underlying_graph_type(::Type{<:AbstractTTN{V}}) where {V}
     return NamedGraph{V}
 end
 
-ITensorNetwork(tn::AbstractTTN) = error("Not implemented")
-
 #
 # Field access
 #

src/treetensornetworks/treetensornetwork.jl

@@ -26,6 +26,15 @@ struct TreeTensorNetwork{V} <: AbstractTreeTensorNetwork{V}
     ortho_region::Indices{V}
 end
 
+# Empty TTN with no vertices. The is-a-tree invariant holds trivially.
+function TreeTensorNetwork{V}() where {V}
+    itn = ITensorNetwork{V}()
+    return TreeTensorNetwork{V}(
+        itn.graph, itn.vertex_data, itn.ind_to_vertices, Indices{V}()
+    )
+end
+TreeTensorNetwork() = TreeTensorNetwork{Any}()
+
 """
     TreeTensorNetwork(tensors; ortho_region=nothing) -> TreeTensorNetwork
 
@@ -69,16 +78,6 @@ end
 const TTN = TreeTensorNetwork
 
 # Field access
-"""
-    ITensorNetwork(tn::TreeTensorNetwork) -> ITensorNetwork
-
-Convert a `TreeTensorNetwork` to a plain `ITensorNetwork`, discarding orthogonality
-metadata. The returned network shares the same underlying tensor data.
-
-See also: [`TreeTensorNetwork`](@ref).
-"""
-ITensorNetwork(tn::TTN) = ITensorNetwork(map(copy, vertex_data(tn)))
-
 """
     ortho_region(tn::TreeTensorNetwork) -> Indices