layer MultiHeadAttention, and show methods for this

src/layers/attention.jl

@@ -74,7 +74,7 @@ struct MultiHeadAttention{P1, D, P2}
   out_proj::P2
 end
 
-@functor MultiHeadAttention
+@layer MultiHeadAttention
 
 function MultiHeadAttention(dims; 
                      nheads::Int = 8,
@@ -83,8 +83,8 @@ function MultiHeadAttention(dims;
                      dropout_prob = 0.0)
 
   dims = normalize_mha_dims(dims)
-  @assert dims.qk % nheads == 0 "qk_dim should be divisible by nheads"
-  @assert dims.v % nheads == 0 "v_dim should be divisible by nheads"
+  dims.qk % nheads == 0 || throw(ArgumentError("qk_dim = $(dims.qk) should be divisible by nheads = $(nheads)")
+  dims.v % nheads == 0 || throw(ArgumentError( "v_dim = $(dims.v) should be divisible by nheads = $(nheads)")
   q_proj = Dense(dims.q_in => dims.qk; bias, init)
   k_proj = Dense(dims.k_in => dims.qk; bias, init)
   v_proj = Dense(dims.v_in => dims.v; bias, init)
@@ -131,3 +131,42 @@ function (mha::MultiHeadAttention)(q_in::A3, k_in::A3, v_in::A3,
   # [α] = [kv_len, q_len, nheads, batch_size]
   return x, α
 end
+
+function Base.show(io::IO, mha::MultiHeadAttention)
+  qk, q_in = size(mha.q_proj.weight)
+  qk, k_in = size(mha.k_proj.weight)
+  v, v_in = size(mha.v_proj.weight)
+  out, v = size(mha.out_proj.weight)
+  # @show q_in, k_in, v_in, qk, v, out
+  print(io, "MultiHeadAttention(")
+  if q_in == k_in == v_in == qk == v == out
+    print(io, q_in)
+  elseif q_in == k_in == v_in && qk == v
+    print(io, q_in, " => ", qk, " => ", out)
+  elseif q_in == k_in == v_in
+    print(io, q_in, " => (", qk, ", ", v,") => ", out)
+  else
+    print(io, "(", q_in, ", ", k_in, ", ", v_in, ") => (", qk, ", ", v,") => ", out)
+  end
+  print(io, "; nheads=", mha.nheads)
+  if mha.q_proj.bias === true
+    print(io, ", bias=true")
+  end
+  if mha.attn_drop.p != 0
+    print(io, ", dropout_prob=", mha.attn_drop.p)  # can't we rename this?
+  end
+  print(io, ")")
+end
+
+Base.show(io::IO, ::MIME"text/plain", mha::MultiHeadAttention) = show(io, mha)
+
+#=
+
+# Test cases:
+
+MultiHeadAttention((3, 4, 5) => (6, 7) => 8; nheads=1)
+MultiHeadAttention(3 => (6, 7) => 8; nheads=1)
+MultiHeadAttention(3 => 6 => 8; nheads=1)
+MultiHeadAttention(8; bias=true)
+
+=#